IZDAJAJO ŽELEZARNA JESENICE. RAVNE, ŠTORE IN INŠTITUT ZA KOVINSKE MATERIALE IN TEHNOLOGIJE LJUBLJANA REVIJA JE PREJ IZHAJALA POD NASLOVOM ŽELEZARSKI ZBORNIK

Navodila avtorjem za pripravo člankov za objavo v reviji Kovine, zlitine, tehnologije

V letu 1992 uvajamo nov način tehničnega urejanja in priprave za lisk revije Kovine, zlitine, tehnologije. Da bi pocenili tiskarske stroške, skrajšali čas od prejema članka do njegove objave in prepustili avtorju končno odgovornost za morebitne neodkrite tipografske napake, smo se v uredništvu odločili, da izkoristimo možnosti, ki jih danes nudi namizno založništvo.

Za oblikovanje in pisanje člankov smo izbrali TgX oziroma IAT]?X sistem, ki je za pisanje tehničnih člankov in knjig v svetu najbolj razširjen. TgX oziroma IATjt;X oblikovalnik besedil je izdelan za sko-raj vse vrste računalnikov, od IBM PC kompatibilnih računalnikov, Apple Macintosh računalnikov, Atarijev, pa do velikih računalnikov. Besedila, oblikovana v IM j.-N-u, so enostavno prenosljiva, saj imajo obliko ASCII zapisa. Kodiranje naših šumnikov je enotno rešeno, tako da lahko pošljete članek, napisan v IAT]?X-u, kamorkoli po svetu, pa z njimi ne bo težav. Zato naprošamo avtorje, če je le mogoče, da napišejo svoje članke z IATpX oblikovalnikom besedil, sicer pa naj nam poleg besedila na papirju pošljejo vsaj disketo z. običajnim ASCII zapisom besedila brez kakršnih koli drugih ukazov za formatiranje.

Vsebina članka

Kako naj članek izgleda vsebinsko, naj si avtorji ogledajo v starih izdajah Železarskega zbornika. Vsak članek pa mora vsebovati:

• slovenski in angleški naslov članka. • imena ler naslove avtorjev, • povzetka v angleščini in slovenščini, • reference, ki naj bodo v besedilu članka označene z

zaporednimi številkami, p r imer 1 - 5 . Način citiranja članka: avtor, inicialkam naj sledi priimek, naslov članka, ime revije, letnik, strani, leto. Način citi-ranja knjige: avtor, naslov, založnik in kraj izdaje, lelo, po potrebi poglavje ali strani.

Besedilo članka naj bo razdeljeno na razdelke (označene z zaporednimi številkami) in po potrebi še na pod-razdelke (označene z decimalno številko, kjer celi del označuje razdelek.

Slike

Vse slike naj bodo na posebnih listih papirja, z jasno označeno številko slike. Slike naj bodo označene z zaporednimi številkami povsod v članku. Originali za vse vrste slik naj bodo ostri in brez šuma. Risbe naj bodo narisane s črnim na belem ozadju. Vse oz-nake in besedila na risbah naj bodo v istem jeziku kot besedilo članka in dovolj velike, da omogočajo po-manjšanje slike na 8 cm. Le izjemoma lahko slika sega čez obe koloni besedila (16.5 cm). Fotografije so lahko katerekoli običajne dimenzije, na svetlečem papirju in

z dobrim kontrastom. Mikroskopska in makroskopska povečevanja označite v podpisu na sliki, še bolje pa z vrisanjem ustrezne skale na fotografiji.

Za vsako sliko naj avtor predvidi, kam naj se slika v besedilu članka uvrsti, kjer naj se nahaja ustrezen podnapis z zaporedno številko slike (na primer: ''Slika 3 prikazuje. . ." , nikakor pa ne: "Na spodnji sliki vidimo. . .") .

Tabele

Avtor naj se izogiba zapletenih tabel z mnogo po-datki, ki bralca ne zanimajo, posebej še, če so isti podatki tudi grafično ponazorjeni. Nad vsako tabelo naj se nahaja zaporedna števila tabele s pojasnilom. Tabele naj bodo povsod v članku označene z zapored-nimi številkami.

Pisanje besedil na računalniku

Avtorje naprošamo, da pri pisanju besedil na računalniku upoštevajo naslednja navodila, saj le-ta pre-cej olajšajo naše nadaljnje delo pri pripravi za tisk:

• ne puščajte praznega prostora pred ločili (pikami, vejicami, dvopičji) in za predklepaji oziroma pred zaklepaji,

• puščajte prazen prostor za vsemi ločili (pikami, ve-jicami, dvopičji)—razen decimalno piko,

• pišite vse naslove in besede z majhnimi črkami (razen velikih začetnic in kratic),

• besedilo naj ne vsebuje deljenih besed na koncu vrstice.

Če avtor pripravlja ilustracije na računalniku, ga naprošamo, da priloži datoteke s slikami na disketo z besedilom članka, s pojasnilom, s katerim programom so narejene.

Pisanje v I^Tj?X-u

Uporabljajte article style. sicer pa se držite vseh I A T E X konvencij. Vse matematične izraze, imena spremenljivk in podobno (razen SI enot) pišite v matematičnem okolju. Uporabljajte že vgrajene fonte. med pripravo za tisk jih bomo zamenjali z ustreznimi PostScript fonti.

Krtačni odtis

Krtačni odtis—končna podoba članka—bo poslan avtorju v končno revizijo. Avtorja naprošamo, da čim hitreje opravi korekture in ga pošlje nazaj na uredništvo. Hkrati naprošamo avtorje, da popravljajo samo na-pake, ki so nastale med stavljenjem članka. Če avtor popravljenega članka ne vrne pravočasno, bo objavljen nepopravljen, kar bo tudi označeno.

Uredništvo

K O V I N E Z L I T I N E T E H N O L O G I J E I z d a j a j o ( P u b l i s h e d b y ) : Ž e l e z a r n a J e s e n i c e , Ž e l e z a r n a R a v n e , Ž e l e z a r n a Š t o r e in Inš t i tu t za k o v i n s k e m a t e r i a l e in t e h n o l o g i j e L j u b l j a n a

I z d a j a n j e K O V I N E Z L I T I N E T E H N O L O G I J E d e l n o s o f i n a n c i r a : M i n i s t r s t v o za z n a n o s t in t e h n o l o g i j o

U R E D N I Š T V O ( E D I T O R I A L S T A F F ) Glavni in odgovorni urednik (Editor) : J o ž e f Arh, d ip l . ing . Uredniški odbor (Assoc iate Editors) : d r . A l e k s a n d e r K v e d e r , d i p l . ing. , d r . J o ž e R o d i č , d ip l . ing. , p r o f . d r . A n d r e j P a u l i n , d i p l . ing. , d r . M o n i k a J e n k o , d i p l . ing. , d r . F e r d o G r e š o v n i k , d i p l . ing. , F r a n c M l a k a r , d ip l . ing. , d r . K a r e l K u z m a n , d i p l . ing. , J a n a J a m a r Tehnični urednik (Product ion editor): J a n a J a m a r Lektorji (Lectors): C v e t k a M a r t i n č i č , J a n a J a m a r Prevodi (Trans lat ions) : p r o f . d r . A n d r e j P a u l i n , d ip l . ing. , d r . N i j a z S m a j i č , d i p l . ing . ( ang le šk i j ez ik ) , J o ž e f A r h , d i p l . ing . ( n e m š k i j e z i k )

N A S L O V U R E D N I Š T V A ( E D I T O R I A L A D R E S S ) : K O V I N E Z L I T I N E T E H N O L O G I J E , Ž e l e z a r n a J e s e n i c e d . o . o . , 64270 J e s e n i c e , S l o v e n i j a T e l e f o n : ( 0 6 4 ) 81 441 T e l e x : 37 219 T e l e f a x : (064) 83 3 9 7 Žiro račun: 5 1 5 3 0 - 6 0 1 - 2 5 7 3 4

Stavek in pre lom: Igo r E r j a v c , T i sk : G o r e n j s k i t isk, K r a n j , Obl ikovanje ovitka: I g n a c K o f o l Fotograf i ja na ovitku: F r a n c i S luga

I Z D A J A T E L J S K I S V E T ( E D I T O R I A L A D V I S O R Y B O A R D ) : P r e d s e d n i k : p r o f . d r . M a r i n G a b r o v š e k , d ip l . i ng . ; č l a n i : d r . B o ž i d a r B r u d a r , d ip l . ing. , p r o f . d r . V i n c e n c Č i ž m a n , d i p l . ing . , p r o f . d r . D. D r o b n j a k , d ip l . ing. , p r o f . d r . B l a ž e n k o K o r o u š i č , d i p l . ing. , p r o f . d r . L a d i s l a v K o s e c , d i p l . ing. , p r o f . d r . J o s i p K r a j c a r , d ip l . ing. , p r o f . d r . A l o j z K r i ž m a n , d ip l . ing. , d r . K a r e l K u z m a n , d i p l . ing. , d r . A l e k s a n d e r K v e d e r , d ip l . ing. , p r o f . d r . A n d r e j P a u l i n , d ip l . ing. , p r o f . d r . Z. Paša l i č , d i p l . ing . , p r o f . d r . Cir i l P e l h a n , d ip l . ing. , p r o f . d r . V ik to r P r o s e n c , d i p l . ing. , p r o f . d r . Bor i s S i che r l , d i p l . ing . , d r . N i j a z S m a j i č , d ip l . ing. , p r o f . d r . J . S u š n i k , d r . L e o p o l d V e h o v a r , d i p l . ing . , p r o f . d r . F r a n c V o d o p i v e c , d i p l . ing .

Po m n e n j u M i n i s t r s t v a za z n a n o s t in t e h n o l o g i j o R e p u b l i k e S l o v e n i j e št. 2 3 - 3 3 5 - 9 2 z d n e 09 . 06 . 1992 š t e j e K O V I N E Z L I T I N E T E H N O L O G I J E m e d p r o i z v o d e , z a k a t e r e se p l a č u j e 5 - o d s t o t n i d a v e k o d p r o m e t a p r o i z v o d o v .

Contents Vsebina

N. Smajič: Computer Simulation and Optimization of VOD Treatment Računalniška simulacija in optimiranje VOD obdelave 313

F. Vodopivec: Microafloying of Steel Mikrolegiranje jekla 319

J. Črnko: The Dependence of the Heat Energy Consumption upon the Working Intensity and the Frequency of the Isolation Maintenance of a Pusher-type Furnace Ovisnost utroška toplinske energije od intenziteta rada i učestalosti održavanja izolacije potisne peči 329

B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt: Tool-steel Wire Dravving at Elevated Temperatures Vlečenje žice iz orodnih jekel pri povišanih temperaturah 333

P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation Odpornost gradbenih jekel proti nastanku in širjenju razpoke 337

M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti 343

M. Jenko, A. Kveder, S. Spruk, L. Koller: Chromizing of Iron Difuzijsko kromanje železa 351

Letno kazalo 357

Computer Simulation and Optimization of VOD Treatment

Računalniška simulacija in optimiranje VOD obdelave

N. Smajič, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana

Mathematical model and the software CAPSS (Computer Aided Produc tion of Stainless Steel) developed as a pad of URP-C2-2566 research program were used for computer simulation of EAF-VOD-CC stainless steelmaking technology line. Basic aim of model testing was to optimise VOD treatment with emphasis on obtaining the maximum productivity at the lowest possible thermal load of VOD ladle and EAF tap temperature. It was concluded that only computer controlled oxygen blowing can secure maximum productivity at the acceptable thermal load of VOD ladle and lowest EA furnace tap temperature.

V okviru petletnega raziskovalnega programa URP-C2-2566 izdelani model in računalniški program CAPSS (Computer Aided Production of Stainless Steel) je bil uporabljen za računalniško simulacijo EOP-VOD-KL tehnologije za izdelavo nerjavnih jekel. Osnovni namen modelnih poskusov je bil optimiranje VOD obdelave s posebnim poudarkom na zagotavljanju maksimalne produktivnosti VOD naprave ob najmanjši možni toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda. Ugotovili smo, da le računalniško programirano pihanje zagotavlja maksimalno produktivnost ob še sprejemljivi toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda.

1 Introduction

The production in metallurgical industries was for a very long t ime based dominantly on experience. However, the development of theory of metallurgical processes and met-allurgical thermodynamics couplcd with small-size and yet povverful computers has made it possible to combine the experience with theoretical knovvledge, which can result in a signilicant improvement of operation per fomiance and raising techno-economic production parameters to an essen-tially higher lcvel.

The application of computers in steelmaking has become indispensable due to high competi t ion and ever increasing demands for lovver priče, better quality and narrower toler-ances of steel propert ies such as strength, ductility, hot and cold deformabil i ty, chemical composi t ion, corrosion resis-tance, etc. The concept of T Q C (Total Quality Control) which excludes any refuse is based on absolutely reliablc control of quality. At present TQC is obligatory for ali high priced products . Consequently, it is imperative also in the manufac ture of stainless steel since the value of a heat of high quality stainless steel can amount up to 300,000 USS. Naturally, there is no room for any allowable refuse despite the fact that technological regulations at present rec-ognize al lowable waste. Conventional technologial regula-tions deal vvith imaginary "average" heat and "normal" con-ditions. For that reason and because of great value, each heat of stainless steel must be regarded as unique i.e., it must be processed in a specific way taking into account actual initial state and condit ions (chemical composit ion, temper-ature, V O D ladle state, pumping sy.stem state, etc.). There is no such thing as standard process parameter. Every de-viation of initial condit ions f rom some imaginary standard can and must be compensa ted for by adequate adaptation of process parameters. Such a complex assignment can suc-cessfully be carried out only by computer aid.

The work was c a n i e d out as a part of the projeet Stain-less Steel vvhich is a imed at the reduetion of production costs, improvement of the quality of stainless steel, devel-opment of new high quality and extra clean e.g. superfer-ritic stainless steels and the opt imizat ion of E A F - V O D - C C tehnological line operation.

The research was concentrated to vacuum oxygen de-carburization rate, which should be inereased as much as possible since V O D treatment is serious bott leneck of the technological line particularly when producing ELI (Ex-tra Low Interstitials) steel. In addition better control of melt temperature and lovver carbon, ni trogen and phospho-rus content especially in čase of extra clean steel must also be considered in any optimization of V O D treatment.

2 Computer simulation and model tests

Main part of the research was carried out in the form of computer simulation of V O D treatment which is a key part of the technological line.

The model tests performed by use of C A P S S (Computer Aided Production of Stainless Steel) vvere undertaken

• to inerease productivity by shortening of V O D treat-ment,

• to improve techno-economic production parameters and

• to determine most rational and expedient measures to be taken in order to improve the present technology

2.1 CAPSS

The PC oriented sof tware C A P S S has been developed in Mathematical Modell ing and Computer Simulat ion De-partment of IMT (Institute of Metals and Technologies) Ljubljana on the basis of a sophisticated mathematical

model elaborated through extensive theoretical studies and i n v e s t i g a t i o n s 1 - 1 0 . The model has been devised primarily for thermodynant ical analysis of the Fe-Cr-C-Si-Mn-Al-Ti-O-N system in ntolten state which is essential in the pro-duct ion of stainless steel.

C A P S S integrates thermodynamic principles, industrial experience, theory of metallurgical processes and expert knowledge required for the economic and massive produc-tion of high quality austenitic, ferritic and superferritic con-ventional , E L C and ELI (Extra Low Carbon, Interstitials) stainless steel.

Thermodynamica l data and published informations on V O D operat ion results were obtained f rom r e f e r e n c e 1 1 - 3 3 . C A P S S vvas calibrated by a posteriori analysis of a number of heats which had been produced in steelvvorks Jesenice to de termine some entpirical unnteasurable constants typical for the s teelworks. C A P S S was developed for:

• Off- l ine control of V O D (Vaeuum Oxygen Decarbur-ization) unit of E A F - V O D - C C technological line for the production of stainless steel

- to reduce the operating costs

- to inerease the productivity and

- to secure the high quality of produced steel

• Research and development purposes

- to pe r fo rm model tests by simulation of V O D operation in order to determine the effect of a change in process variables (oxygen blovving rate, oxygen and argon comsumption, lime ad-dition, etc.) or initial condit ions (e. g. melt composi t ion, temperature, etc.) on operation re-sults (productivity, production costs, quality of steel produced, etc.)

- to opt imize production technology and

- to deve lop new grades of stainless steel

• Educat ion and training purposes

- for training of technical personnel in newly in-stalled V O D units or steelvvorks and

- for educat ion of University students

2.2 Computer control ofVOD treatment

Advantages of the computer control of V O D treatment are nunterous and evident. Firstly, intmediate and continuous inspeetion of the process of vaeuum oxidation and devel-opment of melt temperature. It offers the possibility to stop oxygen blovving at exactly right tirne i.e., at desired final carbon content. The progress and rate of oxidation are con-t i n u o u s ^ moni tored and controlled in order

• to prevent melt temperature f rom exceeding allovvable upper limit,

• to assure the lovvest heat loading of ladle lining and • to attain the m a x i m u m productivity and the lovvest pos-

sible loss of chromium.

Compute r control is benelicial also for reduetion stage of V O D treatment since it

• of fers monitor ing of the reduetion of chromium f rom slag and temperature changes,

• assures exact calculalion of proper addition of lime and reducing means and

• helps to synchronize the operation of whole techno-logical line E A F - V O D - C C and especially of V O D unit and consequent continuous casting machine.

For research and development purposes s imulat ion of imag-inary or real but future heats knovvn also as model test is particularly useful . Similar model tests pe r fo rmed in the form of simulation of real past heats i.e., a posteriory anal-ysis is very instruetive in educat ion and training of technical personnel. It can reveal errors and vvrong decisions made in the past during V O D treatment of a given heat. Simulation and analysis of previous heats can be extensive and include ali heats manufactured e.g. in the last month or year. Such an analysis is valuable for evaluation of the existent tech-nology. It can help to find out and determine oscillations of techno-economic parameters (productivity, specific con-sumption of energy and raw materials , etc.) due to instable technology, unsuitable technical regulations, poor perfor-mance of technical personnel or inadequate maintenance. Particularly valuable is analysis of ex t reme heats i.e., ex-ceedingly good and bad operat ion results. It can result in significant technological improvements and more suitable regulations. By ali means it is first and obligatory step to-vvard the introduetion of computer control of s teelmaking.

3 Optimisation of V O D treatment bv computer s imu-lation

3.1 ELI stainless steel

When producing ELI e.g., superferri t ic stainless steel deni-trogenization of melt proceeds under deep vaeuum (approx. 100 Pa) at a high carbon content. Of course, such a vaeuum can be made only in especial and additional stage of vae-uum treatment vvithout oxygen blovving. There can be one, tvvo or even more denitrogenizat ion steps vvhich depends on initial melt temperature and antount of heat losses. The extent and kinetics of this intermediary denitrogenization treatment can also be determined by model tests. Therefore, the efficiency of one stage and the necessi ty for multistage denitrogenization treatment to obtain required total (C+N) content can be established. As regards the oxygen blovv-ing technique there are tvvo methods namely, un i fo rm and "smar t" oxygen blovving. In final stage of oxidat ion decar-burization rate is very small since the most part of oxygen is uitilized for harmful oxidat ion of chromium. Therefore, it should be smart and useful to decrease oxygen blovving rate gradually tovvard the end of decarburizat ion.

Uniform oxygen blovving is another blovving technique vvhich applies steady blovving rate e.g., 900 m 3 / h and does not take into account rapid and cont inuous inerease in the chromium/carbon activity ratio occur ing in the final stage of decarburization. This change of thermodynamic condi-tions results in a drastically reduced decarburizat ion rate and inereased chromium oxidat ion rate vvhich is accompanied vvith steep rise in melt temperature. Consequently, the final stage of decarburization is mos t delicate since V O D ladle lining must be prevented f rom thermal overloading. On the other side computer s imulat ion in the fo rm of model tests airned at the optimization of V O D treatment should also optimize the productivity. The most appropriate compro-mise betvveen high productivity and temperature limitation can be find out only by the use of computer simulation and model tests.

Model tests carried out by the use of C A P S S vvere planned to compare " smar t " and un i fo rm blovving (900 m 3 / h ) to find out the advantages and deficiencies of each.

"Smart" blowing Uniform 900 m3/h

"Smart" blovving Uniform 900 m3/h

"Smart" blovving Uniform 900 m3/h

Time min. —>

Figure 1. Influence of blovving technique on V O D productivity.

Slika 1. Vpliv načina pihanja na produktivnost V O D naprave.

As can be seen f rom fig. 1 " smar t " blovving character-ized by cont inuously diminishing blovving rate tovvard the end of decarburizat ion, as compared to uni form 900 n t 3 / h rate, results in a serious drop of productivity i.e., prolonga-tion by 40 mins. of the tirne required for decarburization from 1.2% C to 0 .02% C.

Quite unexpectedly, fig. 2 shovvs that adaptation of blovving rate to the continuous change in thermodynamic conditions as applied by "smar t" blovving does not reduce the extent of ch romium oxidation to slag. There is no change in final ch romium content vvhich clearly does not depend on oxygen blovving method.

Uni form and intense oxidat ion vvith 900 m 3 / h blovving rate results in the reduct ion of t ime required for V O D treat-ment by 40 mins. as can be seen f rom fig. 3. Hovvever, this increase in productivity could be too expensive since max-imum melt temperature vvould exceed prescribed 1700°C limit.

Fig. 4 shovvs that blovving technique exerts practically no influence on the vo lume and kinetics of denitrogeniza-tion. Because of a higher melt temperature at uni form blovv-ing favorable influence of temperature on denitrogenization of stainless steel can be seen.

E 3 S o

•C O

17.20

16.80

16.60

16.40

16.20

1450 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0

Time min. --->

Figure 3. Effect of blovving method on melt temperature.

Slika 3. Vpliv načina pihanja na potek temperature taline.

3.2 Conventional stainless steel

The manufacture of ELI stainless steel e.g., superferrit ic steel differs f rom production technology for converttional stainless steel by intermediate stop of oxygen blovving fol-lovved by special usually 15 mins. long denitrogenization step at a high carbon content (approx. 1.0% C). Initial car-bon content of melt planned for the manufac ture of com-

The attention has ben focused at the productivity, extent of ch romium oxidat ion to slag, max imum temperature at the end of oxidat ion and vo lume and kinetics of denitrogeniza-tion. Initial mel t composi t ion and temperature vvas 17.0% Cr, 1.20% C, 0.1%. Si, 0.5%. Mn, and 1550°C, respectively. Final carbon content should be lovver or equal to 0.02 vvt.% . C. V O D treatment should be as short as possible to obtain 1

high productivi ty of V O D unit hovvever, melt temperature should not exceed 1700° degrees. The results of model . tests are presented in figs. 1^}. ^

15

Figure 2. Effect of blovving method on the oxidation of chromium.

Slika 2. Vpliv načina pihanja na obseg in potek oksidacije kroma.

220

0) 3

a E Q)

K

15.60 0 2 0 4 0 6 0 8 0 YoO 120 1 4 0 1 6 0 180 2 0 0 2 2 0

Time min. -- >

1500

A

C O -Q i -<0 O

0 0 Iiiiiii iii iuii lintn li i li i n iii i ii i ii i ii i m i m i iiimiii 111 lil ni hi iiii n lil i iii li ni i ii li ii i ii i n hi i ii' iti ii i ti i m iif

0 15 3 0 45 6 0 75 9 0 105

Time min. — >

"smart" u n i f o r m c o n t r o l l e d

A

E

S o v .

-C O

15.60 liiuinniiiiiniiniimiiniiiiii I 0 15 3 0 4 5 6 0 75 9 0 105

Time min. —> "smart" uniform controlled

F igure 4. Inf iuence of b lowing technique on deni trogenizat ion.

Sl ika 4. Vpliv nač ina p ihan ja na potek in hitrost razdušičenja .

m o n s ta in less steel is t he re fo re s igni f icant ly lower in o rde r to sho r t en V O D t r ea tmen t . Final c a r b o n con ten of c o m m o n steel is h i g h e r w h i c h a lso he lps to reduce the t ime requi red fo r v a c u u m o x y g e n deca rbur i za t ion . Specia l den i t rogen iza -t ion s t age is no t neces sa ry s ince low n i t rogen con ten t is no t p rese r ibed for c o m m o n s ta inless steel . Excep t for E L C g r a d e s t e chno log i ca l p r o e e s s fo r c o m m o n stainles steel does no t i nc lude (tnal v a c u u m degass ing s tage f o l l o w e d b y re-due t i on . T h e r e f o r e v a c u u m deca rbu r i za t ion t rea tment can b e p e r f o r m e d a lso b y con t ro l led b l o w i n g rate in addi t ion to " s m a r t " and u n i f o r m blovving. T h e need fo r addi t ional i.e., con t ro l l ed o x y g e n b l o w i n g appea r s as a c o n s e q u e n c e of vvider r a n g e w h e r e f r o m p roees s p a r a m e t e r s can be se-lec ted . T h e tirst a im of con t ro l led b l o w i n g t echn ique is to m a x i m i z e the p roduc t iv i ty of E A F - V O D - C C p roduc t ion l ine by s h o r t e n i n g d u r a t i o n of V O D t rea tment which is the s lowes t s t age and the re fo re V O D unit acts as a bo t t leneck . T h e p r o b l e m is se r ious par t icu lar ly in čase of U H P electr ic f u r n a c e c o u p l e d wi th V O D uni t . Hovvever, m a x i m i z a t i o n of p roduc t iv i t y by op t imiza t i on of V O D m u s t take into account follovving r e q u i r e m e n t s :

• E A F tap t e m p e r a t u r e should be as low as poss ible , » h ighes t allovvable t empera tu re at tire end of ox ida t ion

is 1700° C and • me l t t e m p e r a t u r e at the end of V O D t rea tment should

c o i T e s p o n d to the requ i ren ten t s of con t inuous cas t ing (CC) .

E x a c t so lv ing of the p r o b l e m is no t poss ib le . A c o m p r o -m i s e is n e e d e d b e t w e e n the con t rad ic to ry r equ i rcmen t s m e n -t ioned . C o m p u t e r s imula t ion i.e., a ser ies of m o d e l tests ca r r i ed out d u r i n g t app ing , ladle t ranspor t and prepara t ion fo r V O D t r ea tmen t is on ly poss ib le . F igs . 5 , 6 and 7 p resen t resu l t s of m o d e l test p l anned to d e t e r m i n e m o s t appropr i -ate con t ro l l ed b l o w i n g t echn ique in order to op t imize V O D t r ea tmen t .

Figure S. Opt imiza t ion of V O D product iv i ty by seleetion of proper b l o u i n g technit jue for c o m m o n stainless steel.

Sl ika 5. Op t imi ran je produkt ivnos t i z izbiro načina p ihan ja pri izdelavi k las ičnega ne r j avnega jek la .

A s seen f r o m fig. 5 " s m a r t " blovving r equ i re s longest t ime for deca rbur iza t ion . T h e r e is no essent ia l d i f f e r e n c e in deca rbur iza t ion t ime at u n i f o r m and con t ro l l ed b lowing t echn ique (101 mins . and 102 mins . , r e spec t ive ly ) .

1 7 5 0

A

O o

a E <]>

1 7 0 0

1 6 5 0

1600

1 5 5 0

1 5 0 0 i

1 450 *—IIHMIlllMI —um—TT r -

0 15 3 0 45 6 0 75 9 0 105

Time min.—>

"smart" u n i f o r m c o n t r o l l e d

Figure 6. Int luenee of b l o u ing me thod on ch romium oxidat ion.

Sl ika 6. Vpliv načina p ihan ja na oks idac i jo k r o m a v žlindro.

In ali three cases chromiunt content at the end of treat-ment is the s a m e as seen in fig. 6. Th i s holds for both oxidat ion and redue t ion stage. Hovvever, it can be seen that dur ing process ing c h r o m i u m content of melt is on the av-erage lovvest at control led b lowing because of lower initial tempera ture .

3 5 0

3 0 0

A 1 1 E 2 5 0 CL 0 .

š 2 0 0

C 0) CD 0 150 i : §

100

" V . 50 LriifiniiHiiiiiiiiiiiiii«

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 160 180 2 0 0 2 2 0

Time min.—>

"Smart" blovving Uniform 900 m3/h

Figure 7. Influence of blovving method on themial load of V O D ladle.

Sl ika 7. Vpliv načina pihanja na toplotno obremenitev V O D ponovce.

At u n i f o r m 9 0 0 m 3 / h oxygen b lowing rate (fig. 7) mel t temperature at the end of oxidat ion jus t exceeded the al-lowable l imit (1700° C). Heat load of V O D ladle is lowest at control led b lowing which is op t imized to obtain praeti-cally the s a m e product iv i ty (102 vs. 101 rnins.) at E A F tap temperature reduced by 20° C.

4 Conc lus ion

By the use of P C or iented so f tware C A P S S which had been developed as a part of the research p r o g r a m m e U R P - C 2 -2566 compu te r s imula t ion of stainless s tee lmaking EAF-VOD techno logy was carr ied out.

A series of mode l tests was per forn ted to invest igate the inf luence of three d i f ferent oxygen blowing methods on the product ivi ty of 9 0 ton V O D unit, ch romium losses with slag, vo lume and kinet ics of deni t rogenizat ion, and themial load of V O D ladle.

Based on the rcsul ts ob ta ined fo l lowing conclusions can be dravvn.

• C o m m o n oxygen b lowing technique k n o w n as " smar t " b lowing does not m a k e it poss ib le to reduce c h r o m i u m losses. Hovvever, it has very negat ive influence on the product ivi ty of V O D unit and consequent ly whole s teelmaking E A F - V O D - C C technological line.

• U n i f o r m oxygen b lowing with cons tant b lowing rate does not inerease c h r o m i u m losses to slag. By proper

seleetion of b lowing rate this b lowing m e t h o d can re-sult in a higher product iv i ty hovvever, V O D ladle lining can be thermal ly over loaded .

• Best results can be achieved by c o m p u t e r control led blovving rate which ensures h ighes t poss ib le p roduc-tivity at lovvest tap tempera ture and thermal load of V O D ladle.

5 References 1 N. Smajič: Mathematical Model for E A F - V O D Stainless

Steelmaking, Proceedings of 6. CENIM, Madrid, October 1985. N. Smajič: Modelle Thermodinamique de 1'Elaboration de 1'Accier inoxydable suivant le Procede VOD, Proceed-ings of 25. JAS, St. Etienne,1986.

N. Smajič: Verifikacija matematičnega modela za računalniško vodenje EOP-VOD tehnologije izdelave nerjavnih jekel, Žel. Zbornik, 1986, 3.

4 N. Smajič: Študij kinetike vakuumskega razdušičenja su-perferitnih talin, Poročilo Metalurškega inštituta v Ljubl-jani, N 89-007. 1989.

N. Smajič: Superferitna nerjavna jekla. Žel. zbornik, 1988.2.

6 N. Smajič: Vakuumsko razdušičenje nerjavnih jekel, Žel. zbornik. 1990, 1.

' N. Smajič: Production D 'Acier Ferritique Inox Avec Car-bone et Azote Tres Bas Par La Technologie EAF-VOD, Bulletin du Cercle de Metaux, 29ems Journees 'Etudes de Metaux, Avril 1990, Saint Etienne.

8 N. Smajič: Vakuumsko razdušičenje nerjavnih jekel, Žel. zbornik, 24, 1990, št. 1.

9 N. Smajič: CAPSS, Demonstration on Technova 92 In-ternational, Graz 1992.

10 N. Smajič: Integralni matematični model EP-VOD tehnologije, Poročilo IMT N. 92-09, Ljubljana, 1992.

11 Mc Coy et al., Electr. Furn. Conf. Proc. 21, 1963, 17-26. 12 Hilty. D.C. cit. v Handbook of Stainless Steel, 1977,

3 -32 . 1 3 Peckner, D.: Handbook of Stainless Steel, Mc Gravv Hill

Book Inc., N.Y. 1977, 3 -26 . 14 Otto J. et al., Stahl und Eisen 96, 1976, 1939-45. 15 Otto J.. Disertation, Aachen 1975. 16 M. Schmidt et al., Stahl und Eisen, 88, 1968, 153. 17 H. Bauer et al, ibid. 90, 1970. 725. 18 C. Wagner: Thermodynamics of Alloys, Addison-Wesley,

Cambridge, 1952. 19 J.F. Elliot et al., Thermochemistry for Steelmaking, vol. 2,

Addison-Wesley, London 1963. 2 0 Ban-ya et al.. Tetsu-to-Hagane, 60, 1974, 1443. 2 1 F. Tsukamoto, Transactions of ISIJ, 26, 1986, 273-81. 2 2 B.I. Leonovič et al., Metalli, 1980, 4. 2 3 Fujio Is hi i et al., Tetsu-to-Hagane 68, 1982, 946-55. 2 4 K. Mori, Transactions of ISIJ, 28, 1988, 246-261 . 2 5 N.P. Vladimirov et al., Metalli, 5, 1973, 89-95 . 2 6 M. Murakami et al., Transactions ISIJ, 27, 1987. 2 7 K. Ishihara et al., Proceedings of 100111 ISIJ Meeting, Oc-

tober, 1980, 836. 2 8 K. Mori et al., Tetsu-to-Hagane, 52. 1966, 1443-45. 2 9 V.P. Luzgin et al.. Izvestija VUZ, Čem. Metal. 9. 1963,

50-54 . 3 0 J.V. Javojskij, Izvestija VUZ, Černaja Met. 1977. 7. 3 1 E.T. Turkdogan et al.. Trans. Metali. Soc. A IME, 1963,

227.

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Microalloying of Steel

Mikroiegiranje jekla

F. Vodopivec, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana

Mechanisms of the effect of microalloying on strength and toughness of steels. Influence on grain size, precipitation hardening, processes of hot deformation, and economv of microallovina with Al Nb, i/, and Ti.

Mehanizmi vpliva mikrolegiranja na trdnostne lastnosti in žilavost jekla. Vpliv na velikost zrn, izločilno utrditev, procesi vroče deformacije in gospodarnost mikrolegiranja z Al, Nb, V in Ti.

1 Introduct ion

The microa l loy ing of steel is a technology which has been intensively deve loped for about 25 years, and it exploits the theoretical know!edge on mechan isms of precipitation hardening, gra in size control , and deformat ion of steel. The term "mic roa l loy ing" is used because steels are alloyed with up to 0 .05% of var ious e lements , and an important influence on the fo l lowing characterist ics and propert ies is achieved:

• austenite and ferri te grain size are diminished, and be-cause of it yield stress, strength, and toughness are in-creased while the ductile/brit t le f racture transition tem-perature is d iminished;

• precipi ta t ion hardening is achieved, this increases the yield stress and strength of steel, d iminishes the tough-ness and increases the ductile/britt le fracture transition temperature;

• the hardenabi l i ty is improved and austenite/ferrite tran-sition tempera ture is lowered;

• the susceptibi l i ty of steel to strain ageing is eliminated;

• the content of d issolved oxygen and sulphur in steel are d imin ished and the puri ty of steel is improved;

• the shape and compos i t ion of non-metal l ic inclusions are changed and the isotropy of properties and the machinabi l i ty of steel are improved;

• the texture in non or iented electrical sheets is improved and the energy losses diminished.

Microa l loy ing e lements are: a luminium, base element for steel deoxida t ion and for the decrease of oxygen is solution, n iob ium, t i tanium, vanadium, zirconium, boron, calcium, te l lur ium, antimony, tin, nitrogen, and in some cases also su lphur and lead. In this paper only microal-loying e lements in the narrower sense will be discussed, i.e. those which inf luence the microstructure, strength and toughness of steel: a lumin ium, niobium, t i tanium, vana-dium, and ni t rogen which are in various combinat ions the basic const i tuents of h igh-s t rength structural steels and mod-em machine-bui ld ing steels. In order to understand better the influence of microa l loy ing e lements on the mechanical properties and the hot vvorking process it is necessary to know the processes and react ions in steel involving these elements, and their c o m p o u n d s with nitrogen and carbon which fo rm precipi tates cal led in the fol lowing as disper-soide phases. The inf luenced processes are austenite grain growth, precipi tat ion hardening in ferrite, and recrystalliza-tion of austenite dur ing hot rolling.

2 Size and stability of austenite gra ins

The first condit ion for the fo rmat ion of small ferri te grains during the cooling of steel are smal l austeni te grains and are obtained either by recrystal l izat ion of austenite af ter hot rolling at a relatively low temperature if a suitable delay of austenite grain grovvth is achieved dur ing the hot vvork-ing, or during the cool ing af ter normal iza t ion. Austeni te grains grow through migra t ion of boundar ies , which can be hindered or s topped if the boundary is p inned to precipi-tates of dispersoide phases. W h e n migra t ion progresses, at first a concavity is fo rmed at the precipi tate, then the grain boundary envelopes it and finally bypasses it. This process requires an additional energy. The dr iving force for the growth is the tendency of material to reach a min imal total energy ( E , ) through the change of the shape and the size of grains, and is obtained by the min imal specif ic surface energy of grains. The total energy consis ts of the vo lume (E v ) and the surface componen t (Ep). Ev is proport ional to the grain vo lume, thus to D3, if D is linear d imens ion of grain, vvhile the surface componen t is proport ional to D2. Schematical ly it can be writ ten Es = I< D2 + I\\D3. The specific energy is thus:

El = 1

D2 D + 1

E.g.: for D = 1, EJD = 2; for D = 2, E,/D = 1.5; for D = 3, Es/D = 1 .33, etc. Thus total energy is the lower the coarses is the grain size.

The prevention of the migra t ion of a grain boundary is achieved when the dis tance among the precipi tates is below a critical value. Instead of the dis tance betvveen the precipitates, which is difficult to be measured , the m o r e easily measurable precipi tat ion size (d) and vo lume part of dispersoide phase ( / ) are used in the analytical t reatment of grain grovvth. The relat ionship be tween the grain s i ze— D, the volume part of p r e c i p i t a t e s — / , and their s i ze—d is according to Zener 1 g iven by the equat ion:

D _ 4d J ~ 3f

The above equat ion vvas fur ther developed for the grovvth of austenite grains in structural steel by Gladmann and Pickering2 . They have assumed that in grain grovvth the energy

D \Z 2 J

is r e l eased . In the e q u a t i o n S — r e p r e s e n t s the b o u n d a r y m i g r a t i o n , y — t h e s u r f a c e e n e r g y of aus teni te , and Z — t h e ra t io betvveen the s ize of a g r o w i n g gra in and the ave rage g ra in s ize in the ma t r ix .

It is e v i d e n t that the gra in g r o w t h is poss ib le on ly if Z > 4 / 3 , o t h e r w i s e the g r o w t h ene rgy c h a n g e is pos i t ive a n d a s p o n t a n e o u s p r o c e s s is not poss ib le . An excep t i on r e p r e s e n t s the ca s e s of ve ry grea t g r o w t h dr iv ing fo rce , e.g. a g ra in s h a p e s w h i c h grea t ly dev i a t e s f r o m the equ i l i b r ium. T h e e q u a t i o n w a s fu r t he r t r a n s f o r m e d into the exp re s s ion e o n n e c t i n g the cr i t ical s ize of p rec ip i ta tes , c/j., with o ther eas i ly n t e a s u r a b l e p a r a m e t e r s :

<k (Wf / 3

2t t V 2

T h e g ra in g r o w t h o c c u r s if the size of p rec ip i t a t es is d > d i-. T h e e q u a t i o n s h o w s that g ra ins g r o w wi th the g r o w t h of the cr i t ical s ize and the dcc rea se of the con ten t of p rec ip i t a t e s , as vvell as w i th the inc reas ing n o n - u n i f o r m i t y of gra in s ize . A s p o n t a n e o u s grovvth is ini t iated the eas ier the g rea te r is the init ial n o n - u n i f o r m i t y of aus teni te gra in s ize . Fo r be t te r u n d e r s t a n d i n g it c a n be m e n t i o n e d that af ter o n e - h o u r of hea t i ng of a Cr -Ni ca rbu r i z ing steel at 9 2 0 ° C , i .e. b e f o r e a n o r m a l g ra in grovvth, the ra t io of m a x i m a l and m i n i m a l g r a in s ize is Z = 3 . 1 8 .

By the s a m e quan t i t y of the d i s p e r s o i d e phase the pre-c ip i t a t e s a re the m o r e e f f i c ien t the smal le r is their size, i.e. the g rea te r is their v o l u m e dens i ty and thus the smal le r is the i r m u t u a l d i s t a n c e . P rec ip i t a t e s are not c o m p l e t e l y s table at the g ra in grovvth t e m p e r a t u r e and g row at p r o l o n g e d an-nea l i ng t inte a n d e spec i a l l y at h igher t empera tu re s loos ing the p i n n i n g e f f i c iency . In s t ruc tura l steel the p rec ip i t a t es of s i zes b e l o w 10 n n i r ep re sen t a low h ind rance for g ra in grovvth b e c a u s e of their ins tabi l i ty c a u s e d by the h igh rat io of s u r f a c e to the total energy .

E f f i c i en t p r ec ip i t a t e s are f o r m e d by d i spe r so ide p h a s e s vvhich are d i s so lved in aus ten i t e at the hea t ing of steel be-fo re the hot ro l l ing o r fo rg ing . As s teels are b e c o m i n g cool , the so lub i l i ty o f d i s p e r s o i d e phase is d i m i n i s h e d , and pre-c ip i t a t e s are f o r m e d vvith s ize d e p e n d i n g o n the t e m p e r a t u r e a n d the l eng th of i so thc rma l annea l i ng .

D u r i n g the t r a n s f o r m a t i o n and the recrys ta l l iza t ion the grovvth rate of ali g r a i n s is not u n i f o r m , s ingle g ra ins grovv fas ter , r e a c h a lovver total energy , b e c o m e m o r e s table , and at su f f i c i en t t e m p e r a t u r e grovv at the e x p e n s e of the i r n e i g h -b o u r s . Th i s is the e x p l a n a t i o n w h y a mic ros t ruc tu r e of g r a i n s o f d i f f e r e n t s ize is f o u n d in n o r m a l i z e d steel vvith a too lovv quan t i t y of the p rec ip i t a t e s for c o m p l e t e p inn ing of the m i g r a t i o n of aus t en i t e gra in bounda r i e s . T h e g ra ins c a n grovv a l so by c o a l e s c e n c e if their space or ien ta t ion is s imi l a r and a re pa r t ed by lovv-anglc bounda r i e s . Th i s o c c u r s in t e x t u r e d ma te r i a l s .

T h e b o u n d a r y m i g r a t i n g at the ex ten t of a n e i g h b o u r g r a in is c o n c a v e . A s impl i f i cd e x p l a n a t i o n is that the a t o m s o n the c o n c a v e s ide are o n a v e r a g e m o r e tinte b o u n d in the c rys ta l lat t ice. T h e mig ra t i on of crys ta l b o u n d a r y is p r o d u c e d by the d i f f e r e n c e in the n u m b e r of a t o m s vvhich are d e p l a c e d o v e r the g ra in b o u n d a r y b e c a u s e of thermal osc i l l a t ion . T h e n u m b e r of j u m p s f ront the c o n v e x to the c o n c a v e s ide of the b o u n d a r y is equa l to the n u m b e r of j u m p s in the r e v e r s e d i r cc t ion , but on the c o n c a v e side m o r e of o sc i l l a t ing a t o m s are re ta ined . Th i s p r o d u c e s a flovv of a t o m s f r o m the c o n v e x to the c o n c a v e s ide, i.e. the sh i f t of c rys ta l b o u n d a r y in the o p p o s i t e d i rcc t ion .

T h e theore t ica l c x p l a n a t i o n fo r the m i g r a t i o n p r o c e s s of a crystal b o u n d a r y tovvards the c e n t r e of c u r v a t u r e is f o u n d in r e f . 3 , vvhere it is s u g g e s t e d that the d r iv ing fo rce fo r the b o u n d a r y m i g r a t i o n is the d e c r e a s e of s u r f a c e energy . T h e rate of mig ra t ion is d e s c r i b e d by a p a r a b o l a of the f o r m

D = Do + A ' 1 / n

vvith D0—an initial g ra in size, D—the g ra in s ize a f te r an annea l ing t ime t, and n—the grovvth e x p o n e n t . T h e o r e t i -ca l ly n is 2 vvhile e m p i r i c a l l y the va lue s betvveen 2 and 4 vvere m e a s u r e d .

T h e c o n n e c t i o n betvveen the g ra in s ize ( D ) and the y ie ld s t ress (RE) is g i v e n by the H a l l - P e t c h e q u a t i o n

RE = RE0 + K D - 1 ' 2

vvith Re0 as a cons t an t d e p e n d i n g o n the c o n t p o s i t i o n and the mic ros t ruc tu r e of steel . T h e c o n s t a n t K is a m e a s u r e for the h inde r ing e f f e c t of c rys ta l b o u n d a r i e s o n the mobi l i ty of d i s loca t ions .

In Fig. 1 t aken f r o m the re f . 5 the r e l a t ion betvveen the gra in size, e x p r e s s e d by D 1 ? 2 a n d the A S T M n u m b e r , and the y ie ld s tress of steel vvith 0 . 1 7 % C and 0 . 8 % M n is shovvn. T h e d e c r e a s e of the g ra in s ize f r o m A S T M n u m b e r 5 to A S T M n u m b e r 10, o b t a i n e d t h r o u g h the m i c r o a l l o y i n g p r o d u c e s an inc reases in y ie!d s t ress of s tee l for abou t 5 0 % . Th i s inc rease takes p l a č e at a n i nc r ea sed t o u g h n e s s and a dec r ea sed duc t i le /br i t t l e f r a c t u r e t rans i t ion t e m p e r a t u r e (Tp) at no tch t o u g h n e s s test. T h e p r o p o s e d r e l a t i on is

^r = T0 + K D - 1 / 2

with To and K c o n s t a n t s d e p e n d i n g o n the c on tpos i t i on and the mic ros t ruc tu re of the s tee l 4 .

Figure 1. Relation betvveen the grain size expressed as D , the ASTM number, and the yield stress of steel vvith 0.17% C and O.S^i.

Mn (Ref.5). Slika 1. Odvisnost med velikostjo zm izraženo kot D - 1 / 2 in ASTM razredom ter mejo plastičnosti jekla z 0.17% C in 0.8% Mn. Po viru5.

T h e m o v e m e n t of d i s l oca t i ons in the lat t ice is h indered by the Pe i e r l s -Naba r ro f o r c e ( r p n ) , a n d e a c h c rys ta l b o u n d -ary p r o d u c e s an add i t iona l obs t ac l e fo r the m o v e m e n t . The

5 10 15 Grain s ize d~ ! / J v mm

10 11 12 13

ASTM number

k p / m m mm"1''

total force (Ts) essential fo r the m o v e m e n t of dislocat ions in polycrys ta l l ine material is:

Ts = Tpn + K D~1'2

A pi l ing-up of d is locat ions at the grain boundary is re-quired in order to accumula te a sufficient driving force for the penet ra t ion of d is locat ion into the ne ighbour grain with a d i f ferent space or ien ta t ion 6 .

3 Disperso ide phases

Dispersoide phases are carhides and nitrides, very fre-quently also carboni t r ides since carbides and nitrides of mi-croal loying e l emen t s are mutual ly comple te ly soluble. The compos i t ion of d i sperso ides depcnds on the amounts of mi-croal loying e lements , n i t rogen, and carbon in steel. If the content of microa l loy ing elements , ni t rogen or carbon is too high, d i sperso ide phases can fo rm already in the melt or dur ing the sol idif icat ion of steel. The size of precipitates in this čase is 100 n m or more , accordingly small is their volume densi ty, and low the h inder ing ef fec t at standard size of austeni te grains. In microa l loyed steel in which the content of mic roa l loy ing elcmenLs for the mos t part does not exceed 0 .05%, the suff ic ient vo lume densi ty of pre-cipitates is not achieved if they are fo rmed in the melt or during the sol idif icat ion. In this čase they are enr iched on the sol idif icat ion in ter faces or in eutect ic clusters, vvhich decreases the ducti l i ty of the steel. Such example represent A1N and N b ( C N ) fo rmed during the solidif ication of steel manufac tu red in electric are fu rnace 7 , vvith a high content of n i t rogen, a lumin ium, and n iobium.

The m e c h a n i s m of grovvth of precipi ta tes involves the solution of small part icles vvith a greater specific surface energy and the d i f fus ion of microa l loying componen t s on coarse, m o r e s table part icles.

The grovvth of precipi tates , o f ten named as Ostvvald ripening, is dese r ibed by the Wagner 8 equat ion

df ~ d30 =

16 aDCV 9 RT

-t

vvith

dt precipi ta te d iamete r af ter the annealing t ime t

do prec ip i ta te d iamete r in t ime 0 u su r face tens ion betvveen the matrix and pre-

cipi ta te D di f fus iv i ty of const i tut ive a toms C concen t ra t ion of const i tut ive a toms V mola r v o l u m e of precipitate R gas cons tan t T absolute t empera ture

The equat ion shovvs that the rate of precipitate grovvth will be at constant o ther condi t ions the faster, the faster is the diffusivi ty, the greater is the concentra t ion of consti tu-tive atoms in solut ion, and the h igher is temperature. Thus the ideal d i sperso ide is that vvith the lovvest solubility of constituents, and vvith the lovvest d i f fus iv i ty of microal loy-ing element , since the d i f fus iv i ty of n i t rogen and carbon in interstitial solution is very fast .

Let us a s sume that the steel conta ins 0 .03% A1N vvhich ensures the austenite grains size af ter normal iza t ion of 6 -7 ASTM n u m b e r 9 . Fig . 2 p iesents the calculated A1N

precipitate size for such a steel af ter one hour fiolding at var ious temperatures , the content of a l u m i n i u m nitride, the vo lume densi ty of precipitates, and the re la t ive austenite grain size. The solubili ty product used for the calculat ion is in good agreement vvith the A1N solubil i ty de te rmined for Cr-Ni carburiz ing steel9 . If the heat ing tempera ture of steel is inereased f r o m 900 to 950° C, the s a m e hinder ing ef fec t can be obtained vvith an about 4 0 % higher content of nitride, vvhile above 1000°C the p inning e f fec t of a lumin ium ni tr ide is very rapidly d iminished.

0.15

i.0,10 300

200

100

,0.05

OL o

r 30

1000 1100 T e m p e r a t u r e °C

F i g u r e 2 . Relation betvveen the annealing temperature and the precipitate size. the number of precipitates per unity of volume, the content of A1N in solution, and the austenite grain size. The

theoretical AIN content is 0 .03%. The calculation is based on the

A1N solubility product given in ref.2 . Sl ika 2. Odvisnost med temperaturo žar jenja in vel ikost jo izločkov, številom izločkov na enoto prostornine, kol ičino AIN v raztopini in velikostjo zm austenita. Teoretična vsebnost AIN 0.03%. Izračun je

izvršen na osnovi topnostnega produkta za AIN v viru2 .

In microal loyed steel usual ly there are 2 or 3 grain grovvth inhibitors; AIN, Nb(CN) , T iC and VN. The p resence of precipi tates fo rmed by the addit ion of 0 .03% n iob ium to the steel vvith 0 . 1 0 - 0 . 2 0 % C ensures gra in s izes of A S T M number 10-11 after normal iza t ion .

The most f requent d ispersoide is a lumin ium nitr ide (AIN) vvhich is found in ali steels deox id ized , and thus mi-croal loyed vvith a luminium. T h e solubil i ty of AIN and of other d ispersoide phases in austeni te in s truetural steels is given by the solubili ty product . Ref . 1 0 g ives the follovving solubili ty product for a lumin ium ni tr ide

l o g ( A l x N ) = - 6 7 7 0 / T + 1 .48

In the above equat ion N and Al represent the vveight content of both e lements in solut ion in the steel, and T is the tempera ture in K. Accord ing to ref. 1 0 , 1 1 , and solubili ty products for other d isperso ide phases are

12 the

log(Ti x C) = — 1 0 4 7 5 / T + 5 . 3 3 ti

log(Ti x N ) = - 8 0 0 0 / T + 0 . 3 2 12

log (V x C) = - 9 5 0 0 / 7 " + 6 . 7 2 11

l o g ( V x N ) = - 8 3 3 0 / T + 3 .46 10

l o g ( N b x C + N ) = - 6 7 7 0 / 7 " -f 2 . 2 6 10

In s o m e r e f e r e n c e s a l so o the r e q u a t i o n for the solubi l i ty of d i s p e r s o i d e s is f o u n d but they d o no t d i f f e r s ign i f ican t ly f r o m the a b o v e g iven .

T h e so lub i l i ty o f ali the d i spe r so ides , bu t of v a n a d i u m c a r b i d e , in aus ten i t e w i th up to 0 . 2 % C and 0 . 0 1 % N is sma l l a n d at the n o r m a l i z i n g t e m p e r a t u r e less than 10% of the q u a n t i t y at the t e m p e r a t u r e of about 1200° C . O n the con t ra ry , the so lub i l i t y of v a n a d i u m ca rb ide in aus ten i t e is ve ry h igh , and th is d i s p e r s o i d e is c o m p l e t e l y d i s so lved a l r e a d y at abou t 9 0 0 ° C in steel wi th 0 . 2 % C. O the r d i sper -s o i d e s a re v e r y s tab le b e c a u s e of the l ow solubi l i ty at the n o r m a l i z i n g t e m p e r a t u r e s a n d the inh ib i t ion of gra in g r o w t h is d i m i n i s h e d o n l y at h i g h e r t empe ra tu r e s .

D u r i n g the c o o l i n g f r o m the solubi l i ty t e m p e r a t u r e and at i s o t h e r m a l h o l d i n g d u r i n g s u c h coo l ing the f o r m a t i o n of p rec ip i t a t e s is ve ry s low (Fig . 3) d u e to s low f o r m a t i o n of n u c l e a t h o u g h the sol id so lu t ion is h igh ly o v e r s a t u r a t e d 1 3 . T h e e x p l a n a t i o n fo r the s l o w f o r m a t i o n of nuc lea is the g rea t d i lu t ion s ince the c o n t e n t se ldon t e x c e e d s 0 . 0 3 % vvhich e.g. r e p r e s e n t s 3 a t o m s of t i t a n i u m per 10000 a t o m s of i ron. T h e n u m b e r of a t o m s of m i c r o a l l o y i n g e l e m e n t s is thus ve ry lovv a n d c o n s e q u e n t l y the rate of f o r m a t i o n of suf f ic ien t stat is t ic a g g r e g a t i o n s of a t o m s f r o m vvhich p rec ip i t a t ion nuc lea are f o r m e d is v e r y slovv. T h e k ine t i c s of the prec ip i ta t ion d u r i n g the h o l d i n g a f t e r d i rec t c o o l i n g f r o m the solubi l i ty t e m p e r a -ture is a slovv p a r a b o l a h igh ly d i f f e r en t f r o m that de sc r ib ing the f o r m a t i o n of p r ec ip i t a t e s in aus ten i t e q u e n c h e d f r o m the so lub i l i ty t e m p e r a t u r e and then r e h e a t e d (Fig. 3). The k i -ne t i c s of A1N f o r m a t i o n is in this čase a s tep pa r abo l a indi-ca t i ng that the rate of grovvth of p rec ip i ta tes is d e t e r m i n e d b y the d i f f u s i o n ra te of a l u m i n i u m on the n u c l e a f o r m e d d u r i n g the r e h e a t i n g of s tee l , d u e to the h igh ove r sa tu ra -t ion b e c a u s e of the c o o l i n g to a m b i e n t t e m p e r a t u r e o r to the d o u b l e t r ans i t ion of the aus ten i t e / fe r r i t e phase b o u n d a r y o n vvhich the so lub i l i ty of A1N is c h a n g e d s t rongly.

d i u m (Fig. 4). T h e spec i f ic vveight of n i o b i u m c a r b i d e is h ighe r than that of a l u m i n i u m ni t r ide , the vveight so lubi l i ty of bo th in aus ten i t e is s imi lar , t hus the s a m e vveight con -tent of n i o b i u m in aus t en i t e g i v e s less p rec ip i t a t e s . C o n s e -quent ly , if s e e m s p r o b a b l e that n i o b i u m h i n d e r s the m i g r a -t ion of b o u n d a r i e s a lso by s o m e o the r m e c h a n i s m , e.g. by a s eg rega t ions on gra in b o u n d a r i e s vvhich p r o d u c e s a g rea te r n u m b e r of p rec ip i t a t e s o n these b o u n d a r i e s as it c o u l d be e x p e c t e d f r o m the a v e r a g e n i o b i u m c o n t e n t in steel . Re f . 1 4

presen t s m i c r o g r a p h i e s shovving that the b o u n d a r i e s or sub -b o u n d a r i e s of aus ten i t e g r a i n s are m a r k e d vvith s t r ings of p rec ip i ta tes vvhich c o n f i r m the poss ib i l i ty of an in te rc rys -ta l l ine seg rega t ion of n i o b i u m . T h e s ize of aus t en i t e g ra ins is thus re la ted to the t h e r m a l d e f o r m a t i o n h i s to ry of steel. R e f . 1 5 de sc r ibes a b i m o d a l s ize d i s t r i bu t ion of p rec ip i t a te a f te r the ro l l ing of n i o b i u m steel f r o m 1 0 5 0 ° C vvhich c a n a lso b e e x p l a i n e d s u p p o s i n g an in te rc rys ta l l ine s eg rega t i on of n i o b i u m .

3 0

o . u o

20

10

\

Č 4320

~ ~ \ \ ^ ^

-V

i * - —a

Nb

0,05 0.10 C o n t e n t o f N b a n d V . %

0.15

F i g u r e 4. Relation betvveen the amounts of niobium and vanadium in steel and the size of austenite grain after half-hour and 8-hour

austenitizing at 920°C. Basic steel composition: 0.18% C, 0.95% Mn, 0.28% Si, 1.0% Cr and, belovv 0.002% Al (Ref.16). S l i k a 4. Odvisnost med količino niobija in vanadija v jeklu in

velikostjo zm austenita po polurni in 8 umi austenitizaciji pri 920°C. Osnovna sestava jekel: 0.18% C, 0 .05% Mn, 0.28% Si, 1.0% Cr. pod

0.002% Al, . Po viru16 .

F i g u r e 3 . Kinetics of A1N precipitation after various thermal history of steel vvith 0.11% C, 0.49% Mn, 0.029% Al, and 0.0063% N.

S l i k a 3 . Kinetika precitipacije A1N po različni temiični zgodovini jekla z. 0.11% C. 0.49% Mn, 0.029% Al in 0.0063% N.

It m u s t b e m e n t i o n e d that n i o b i u m if its concen t r a l ion e x c e e d s a b o u t 0 . 0 3 5 % and at h igh n i t rogen c o n t e n t s — t h e l imit is at a b o u t 0 . 0 1 2 % , is b o u n d d u r i n g the so l id i f ica t ion p r o c e s s in to a c a r b o n i t r i d e ve ry r ich in n i t rogen and prac t i -ca l ly u n s o l u b l e d u r i n g hea t i ng the steel b e f o r e the ro l l ing 7 . N i o b i u m b o u n d in this p h a s e is lost as ac t ive mic roa l l oy -ing e l e m e n t , t h u s the m i c r o a l l o y i n g vvith n i o b i u m in steel m o l t e n in e lec t r i c are f u m a c e is e c o n o m i c a l on ly u p to about 0 . 0 3 % . In C r M n č a s e h a r d e n i n g steel by a l ready 0 . 0 2 % N b the s a m e s tab i l i ty and s i ze of aus ten i t e g ra ins is ach i eved as vvith the s a m e a m o u n t of a l u m i n i u m or vvith 0 . 1 % vana -

4 M i c r o a l l o y i n g a n d p r e c i p i t a t i o n h a r d e n i n g

T h e p rec ip i t a t ion h a r d e n i n g is o n e f o r m of d i s p e r s i o n ha rd -en ing , i.e. h a r d e n i n g c a u s e d by a nevv p h a s e vvhich is f o u n d in smal l quan t i t i e s in the me ta l l i c m a t r i x . T h e gene ra l ex-p ress ion desc r ib ing the r e l a t i ons betvveen the quan t i t y of p rec ip i t a t es ( / ) , their s ize (d), the shea r m o d u l u s ( G ) , the Burgers vee to r of d i s l o c a t i o n s (b), and i ne rea se of s t rength ( A R t ) w a s p r o p o s e d by H o m b o g e n 1 7 in the follovving f o r m

ARr = A - 9 2 1 d

K is a cons t an t vvith a v a l u e I \ = 1 f o r a po lyc rys t a l l i ne ma te r i a l a n d u n i f o r m l y d i s t r ibu ted s p h e r o i d a l par t i c les of the nevv phase . T h e h a r d e n i n g is p r o p o r t i o n a l to the third roo t of the quan t i ty of p rec ip i t a t ed p h a s e a n d inverse ly pro-por t iona l to the par t ic le s i ze of tha t phase . It is thus m o r e s t rongly d e p e n d a n t o n the s ize than o n the quan t i t y of pre-c ip i ta tes . T h e p rec ip i t a t i on h a r d e n i n g is s t ab le o n l y till the shear m o d u l s is no t d i m i n i s h e d b e c a u s e of the t e m p e r a t u r e c h a n g e or the p rec ip i t a t e s d o no t h i n d e r the m o v i n g of dis-loca t ions . In m i c r o a l l o y e d s tee l the p r ec ip i t a t e s f o r m e d at

0 . 0 2 9 A I , 0 , 0 0 6 3 N

1 3 0 0 < C . 2 n - 2 0 ° C — 8 4 0 ° C

1 3 0 0 ° C . 2 h - 2 0 ° C — 1000°C 1 3 0 0 ° C , 2 h — 8 4 0 ° C

1300 °C, 2 h —1000°C

the nornializing temperature, which are a very efficient hin-drance for grain grovvth, do not cause precipitation hard-ening. This would be obtained only by a much greater number of precipitates, at least for one order of magnitude greater than it is usually found in microalloyed steel. In such a čase the ductility and the toughness of steel would be diminished.

The highest precipitat ion hardening of microalloyed steel is obtained if precipitates are formed below about 620°C when the shear modulus of- ferrite is high enough, and the internal stresses due to the formation of coherent precipitates are not relaxed. Coherent precipitation occurs by carbonitr ides, carbides, and nitrides of niobium, vana-dium, and t i tanium which have a cubic crystal lattice, but not by the hexagonal a luminium nitride which produces thus virtually no precipitat ion hardening of ferrite. The lattice parameter of cubic precipitate is different than that of ferrite. Both lattices can accommodate by elastic deformation and the intemal stresses on the contact surfaces are proportional to the hardening, generally called as coherent hardening. The stress field around the precipitates hinders the move-ment of dis locat ions in a greater volume of matrix than the precipitate alone. At increasing size of precipitates the co-herence is lost and the boundary between the precipitates and the matr ix becomes an actual phase boundary vvithout elastic accommodat ing stresses. The hardening is achieved only by hindering of dislocations moving at plastic defor-mation. This hardening is called a dispersion hardening and it can be calculated according to the Hornbogen equation. In this type of hardening ali carbide and nitride phases, in-cluding a luminium nitride and cementi te, behave in equal way, and the effect depends on the amount and the size of precipitates.

The moving dislocation can cut small precipitates vvith-out stress field . The critical size of precipitate depends mainly on their shear modulus , e.g. for copper precipitates in ferrite the critical size is about 10 n m and, for TiN precip-itates only 3 nm. Precipitation hardening of microalloyed steel is relatively strong. For the evaluation of the hardening effect of n iobium carbonitr ide the follovving semiempirical expression was developed by Yeo and covvorkers19

ARe = l ^ V o N b 1 ' 3 0.12)

£ e

cc <3

300

200

100

^ A R e = f(d).Nb \ ARe = 1 5 / 5

\ d \

= 0.03 7.

(7oNb,/3-0.12)

\ \ \ \ \ \ AR = f (7<>Nb), d =0.C )5pm

with Nb as n iobium content in vveight %, d—the size of niobium carboni t r ide precipitates, and ARe—the increase of yield stress.

The exponent at the niobium content proves that the expression was der ived through simplification of the Horn-bogen equat ion. A disadvantage of the expression is the lack of parameters considering the temperature of forma-tion of precipitates and the shear modulus, thus it can be used only for heat treatment by quenching and ageing at a selected temperature.

Fig. 5 presents the influence of precipitates size at con-stant n iobium content , and the content of niobium at con-stant size of 50 n m precipi tates on the hardening effect. Already a small amount of n iobium is efficient if present in steel in small precipitates. The increase of the content of niobium does not improve the hardening effect to an economica!ly justif ied extent. Fig. 5 further proves that precipitates of an average size of 25 nm, vvhich can be found in microal loyed steel after normalizing, cause hardly a hardening effect .

Practically only a part of precipitat ion hardening effect can be industrially exploited hovvever it is not negligible

0 0.01 0.02 Precipitates size d. pm

t t i i 0 0,02 0,04 0,06

Content of Nb,% F i g u r e 5 . Relation betvveen the size of precipitates in steel vvith

0 .03% Nb or N b C concentrat ion in 5 nm precipitates, and the increase of yield stress.

S l i k a 5. Odvisnost med vel ikost jo izločkov v jeklu z 0 .03% N b oz. količino NbC v izločkih z vel ikost jo 5 nm in povečanjem meje

plastičnosti.

f rom the vievvpoint of the material strength. E.g. a small change in basic composi t ion of the steel vvith a yield stress above 350 N / m m 2 , and microal loying vvith n iob ium and vanadium can give a yield stress above 470 N / m m 2 , vvhere precipitation hardening due to format ion of vanad ium car-bide during the cooling of steel af ter normal iz ing contributes for about 50 N / m m 2 . As already ment ioned, the precip-itates formed in the approximate temperature range 570 to 620° C are efficient. At lovver temperatures the format ion of precipitates is too slovv due to the slovv d i f fus ion of vana-dium, and it could be exploited only by a longer annealing or in a very slovv cooling vvhich is economic only in coils. The effects of the diminut ion of grain size and of the pre-cipitation hardening on the yield stress are additive, their influence on the other tvvo very important properties, the notch toughness and the ductile/brittle fracture transition temperature, is opposite. Diminished grain size increases the toughness and decreases the transition temperature vvhile the precipitation hardening has an opposi te effect . Fig. 6 presents, according to data in ref .4 , some relat ions vvhich confirm the above conclusions for a standard as normalized Nb-V microalloyed steel. By thermal treatment, e.g. by normalizing and through the rate of cool ing, a rather dif-ferent relation betvveen the yield stress and the toughness transition temperature can be achieved, even an unaxcept-able transition temperature can be obtained vvhich nullifies ali the advantages of microal loying.

5 Microalloying and hot deformat ion

Most steel products are manufac tured by hot rolling vvhen the steel cross section is reduced f rom pass to pass at drop-ping temperature till a final thickness of plate, strip or bar is obtained. A similar sequence of events is found by forgirig only the sequence of partial deformat ions is less uniform. During the rolling process the steel is cooled partially by radiation and the convect ion into the surroundings, and par-tially by contact vvith the cool ing vvater, rolls, hammers or

4 0 0

3 0 0

E E

• 200 t r

100

100 A R e N / m m 2

: 17,36 + 1.81 D"v2

Re = f ( d )

250 r+80

V PT= f (IU)

T T = f (D) 0,15 C, 0,46 Si. 1,16 Mn , 0 036 V,

0,015 Nb normahsed

L I i

12 10 6 ASTM n u m b e r 4

+60

+40

+20

-4 o X

20

-I -40

o o

O) i _ ZD a O) CL E (D

C O Cn c a

-60

20 40 6 0 80

G r a m s i z e D , jjm

100

R e l a t i o n s h i p s g r a i n s i z e - y i e l d s t r e s s g r a i n s i z e - t r a n s i t i o n t e m p e r a t u r e p r e c i p i t a t i o n h a r d e n i n g -

- t r a n s i t i o n t e m p e r a t u r e

F i g u r e 6 . Rela t ion b e t w e e n the grain size in as normal ized steel, the prec ip i ta t ion ha rden ing , and the yield stress increase due to the

prec ip i ta t ion h a r d e n i n g ( A R e ) , and the notch toughness t rans i t ion t empera tu re (PT).

S l i k a 6 . Odv i snos t m e d ve l i kos t j o kr is ta lnih z m v no rma l i z i r anem jek lu , oz . iz loč i lno u t rd i tv i jo in m e j o plast ičnost i p o v e č a n o zaradi iz loči lne u t rd i tve (ARe) ter p r e h o d n o t empera tu ro ži lavost i (PT).

other cooler parts of equipnient . Because of the successive deformat ions the steel is in de fo rmed state for some time and it contains a great number of point and line defects. The rate of d i f fus ion processes in the deformed ntatrix is very fast. Jonas and covvorkers21 2 2 found that the nucleation rate of precipitates was for an order of magnitude faster during the deformat ion , and that the rate of precipitates growth in de fo rmed austenite was for two orders and during the de-format ion even for three orders of magnitude greater than in not de fo rmed or in recrystallized austenite. The static recrystall ization vvhich el iminates f rom austenite the defor-mat ion energy delayed for a few seconds corresponds thus to a 100 or even 1000 sec. long annealing. Any component which delays the recrystallization thus highly accelerates the proeess of precipitaton but only as long as austenite remains unrecrystall ized. As soon as the recrystallization is finished the rate of precipitation is diminished again. E.g. in labo-ratory rolling of 12 m m plates f rom 60 m m billets the steel remained be tween the rolls for 0.47 seconds, and total time of rolling was 70 seconds. Let us assume a great rate and an un i fo rm precipitat ion in the period when steel is deformed betvveen the rolls. The rate of precipitates growths is de-seribed by approximate cubic parabola which can be simpli-fied for a rough evaluation into the expression t / ' / ' 5 % Kt, t being the time. The calculation shows that the ratio of precipitates size in unrecrystall ized austenite ( d a n ) and in recrystall ized austenite ( d a r ) is dan/dar = 4.5. If steel is rolled by uncompleted intetpass recrystallization and if it contains a small quanti ty of microalloying elements the

unhomogenity of microstructure represented by the num-ber of anormally grown grains is the greater the lower is the rolling temperature 9 , 2 because of the unhomogeni ty of precipitation during the rolling. During the rolling of low and medium alloyed steel wi th an austenite microstructure static recrystallizaton is the basic proeess for the elimina-tion of deformat ion energy. Dynamic sof tening processes and static recovery are virtually negligible. In absence of interpass recrystallization static recovery rapidly eliminates the deformation hardening, and it does not change the size of austenite grains. Niob ium is the microal loying element which has the strongest delaying effect on the rate of static recrystallization of austenite. Two explanat ions are pro-posed for the mechan ism of the effect of niobium. Tite first elaims that the effect is l inked to n iobium in solid solu-tion in austenite2 4 . The proeess of static recrystalization is initiated on the grain boundaries , it seems thus that the inhi-bition of formation of recrystall ization nuelea on boundaries is connected to the presence of n iob ium at these boundaries. Fig. 7 shows that the temperature of completed interpass static recrystallization of austenite is already by 0.02% nio-bium inereased for about 100° C in the C r M n carburizing steel. The weight content of 0 .02% of n iobium means that the solution tn austenite contains appr. 1.1 n iobium atom per 104 iron atoms. A logic conclusion is that sueh a di-lution could hardly influence the proeess linked to shifts of iron atoms and it seems justif ied to conclude that the austenite grains boundaries are richer in n iobium due to a segregation. This explanation is supported also by the fact that small amounts of n iobium improve the nardenabili ty of steel through the delaying the nucleat ion of ferrite below the t ransformation temperature. Thus n iob ium can hinder the formation of recrystallization nuelea and ferrite by a similar mechanism.

o:

C r M n cc ise harder

R o

r i n g steel C r M n cc

u - O O 6

G -

/ i P

— m —

/ / /

/ ' / F

/

^

6 E E

in cz 4 o

t OT

O! o*a U . V - 10 1200 1100 1000 900 800

Init ial rolling t e m p e r a t u r e , ° C

F i g u r e 7. Inf luence of the initial roll ing t empera tu re on the ratio length/width (R) and on the n u m b e r of unrecrys ta l l i zed aus teni te grains (P). Steel G : 0 . 1 4 % C . 1% M n , 0 . 8 5 % Cr, 0 . 0 2 % Nb, and 0 . 0 0 7 8 % N; steel F: 0 . 1 6 % C , 1 .1% M n , 0.98%: Cr, 0 . 0 2 5 % Al,

and 0 . 0 0 9 5 % N. S l i k a 7 . Vpliv zače tne t e m p e r a t u r e va l j an ja na r azmer j e

dolž ina/š i r ina (R) in na š tevi lo nerekr i s ta l iz i ran ih z m austeni ta (P). J ek lo G: 0 . 1 4 % C, 1% M n , 0 . 8 5 % Cr, 0 . 02% N b in 0 . 0 0 7 8 % N;

j e k l o F: 0 . 16% C, 1.1% M n , 0 . 98% Cr, 0 . 0 2 5 % Al tn 0 . 0 0 9 5 % N.

The second hypothesis links the influence of niobium on the recrystallization on precipitates formed during the deformation. Two questions are not explained by this hy-

po thes i s : w h y the p rec ip i t a t e s of o the r mic roa l l oy ing ele-m e n t s , e .g . T i C , and V N a n d A1N, vvhich are also f o r m e d du r ing the d e f o r m a t i o n , h i n d e r the s tat ic rec rys ta l l i za t ion of aus ten i te to a m u c h lesser e x t e n t (F ig . 8), and w h y the r ec rys ta l l i za t ion p r o c e s s t akes p l a č e w h e n the con ten t of n i o b i u m in so l id so lu t i on is d i m i n i s h e d b e l o w a l imit of about 0 . 0 0 5 % d u e to the f o r m a t i o n of prec ip i ta tes . Bo th e x p l a n a t i o n s of the e f f e c t of n i o b i u m are f o u n d in recent p a p e r s o n m i c r o a l l o y i n g , a n d it is lef t to the r eade r to c h o s e the m o r e p r o b a b l e s i g n i f i c a n c e we igh t i ng the s ign i f i cance of e m p i r i c a l f ind ings .

300

0,050 0,100 0.150 0.200

Content of e lemen ts . 0 / «

0.250

Figure 8. Influence of the content of various microalloying elements on the hindering temperature of static recrystallization of austenite.

Slika 8. Vpliv vsebnosti raznih mikrolegimih elementov na temperaturo zaustavitve statične rekristalizacije austenita.

In F i g . 9 the e f f e c t of ro l l ing t e m p e r a t u r e on the con -tent of A1N and N b C in v a r i o u s s teels , and on the ferr i te grain s ize g i v e n as in te rcep t l eng th by ro l l ing 15 m m pla tes f rom 60 m m bi l l e t s in 7 p a s s e s is s h o w n . Ali the s teels were h e a t e d to 1 2 0 0 ° C b e f o r e the rol l ing. In steel vvith-out n i o b i u m w h e r e the in te rpass rec rys ta l l i za t ion of aus ten-ite is fas t a n d c o m p l e t e , on ly f e w prec ip i ta tes are f o r m e d during the ro l l ing a n d the i n f l uence of t e m p e r a t u r e o n the arnount of p r e c i p i t a t e s is h a r d l y p e r c e i v a b l e b e c a u s e at de-creasing ro l l i ng t e m p e r a t u r e the con ten t of A1N f o r m e d dur -ing the ro l l ing is ve ry s l o w l y ine reased . T h e p rec ip i t a t ion behav iour in n i o b i u m steel is s ign i f i can t ly d i f f e r en t b e c a u s e austenite r e m a i n s b e t w e e n p a s s e s fo r l onge r t ime unrec rys -tallized, o r the q u a n t i t y of aus ten i t e w h i c h d o e s no t re-crystal l ize at ali betvveen pa s se s is ine reased , and thus the precipi ta t ion is fas ter . B e l o w a l imit t e m p e r a t u r e aus teni te remains c o m p l e t e l y un rec rys t a l l i z ed b e t w e e n passes and the precipi ta t ion is a c c e l e r a t e d to sueh ex ten t that prac t ica l ly ali A1N and N b C are p r ec ip i t a t ed in the re la t ive ly shor t ro l l ing time of 1 m i n u t e .

On the b a s e o f the p r o c e s s e s of recrys ta l l i za t ion and of p rec ip i ta t ion tvvo t e c h n o l o g i e s of rol l ing of m i c r o a l l o y e d steel w e r e d e v e l o p e d . In t h e r m o m e č h a n i c a l rol l ing the s labs are rol led to a t h i c k n e s s w h i c h is 3 0 - 5 0 % grea te r than the final t h i cknes s of p l a t e s , the ro l l ing is s t opped till steel t em-perature d r o p s belovv a b o u t 9 5 0 ° C , and then the p la tes are rolled to the f inal t h i c k n e s s in scve ra l pas ses , the n u m b e r de-pends on the s t r eng th of the ro l l ing s tand , and coo led in air. D e f o m i e d aus t en i t e is d u r i n g the e o o l i n g ve ry rap id ly t rans-formed into f i n e g r a i n e d fer r i te and p e a r l i t e 2 5 , whi le A IN and NbC prec ip i t a t es h i n d e r the g r o w t h of fer r i te g ra ins a f te r the t r ans fo rma t ion . A finegrained m i c r o s t r u c t u r e wi th h igh strength and t o u g h n e s s is ob t a ined , and i! s u p p o r t s the p re -cipitation h a r d e n i n g wi th V C d u r i n g the air eoo l ing of p la te s with an a c c e p t a b l e de te r io ra t ing e f f e c t o n n o t e h toughness .

0 _L _ L 1200 1100

Initial rolling 1000 900

temperature , °C

Figure 9. Relation betvveen the initial temperature of tvvo steel vvith a similar basic composition, one microalloyed vvith niobium, the amounts of A1N and NbC precipitated during the rolling and the

grain size after air eooling. Slika 9. Odvisnost med začetno temperaturo dveh jekel s podobno

osnovno sestavo, eno pa mikrolegirano z niobijem na količino A1N in NbC, ki sta nastala med valjanjem in na velikost zrn po ohladitvi na

zraku.

Thi s t e c h n o l o g y is app l i ed fo r s tee l m i c r o a l l o y e d vvith n io -b i u m and v a n a d i u m vvhich c a n a c h i e v e y i e ld s t resses up to 5 0 0 N / m m 2 at e a r b o n c o n t e n t s belovv 0 . 1 5 % . S imi la r p roper t i e s are ob t a ined vvith the c o m b i n a t i o n of ro l l ing in less con t ro l l ed t e m p e r a t u r e r a n g e a n d n o r m a l i z a t i o n annea l -ing. Fig. 10 r ep r e sen t s the v a r i o u s h a r d e n i n g m e c h a n i s m s in n o r m a l i z e d steel m i c r o a l l o y e d vvith a l u m i n i u m , n i o b i u m , and v a n a d i u m .

T h e a d v a n t a g e s of m i c r o a l l o y i n g c a n b e e x p l o i t e d also t h rough the ro l l ing p r o c e s s vvith c o n t r o l l e d rec rys ta l l i za t ion . Th i s m e t h o d d e m a n d s an exac t h a r m o n i z a t i o n of s teel c o m -pos i t ion vvith the d e g r e e of d e f o r m a t i o n a n d t he p a s s se -q u e n c e , s ince the t e m p e r a t u r e a n d the pe r p a s s d e f o r m a t i o n m u s t enab le the c o m p l e t e i n t e rpa s s r ec rys t a l l i za t i on , and si-m u l t a n e o u s l y also the f o r m a t i o n of p r ec ip i t a t e s vvhich h in -de r the grovvth of r ec rys t a l l i z ed aus t en i t e g ra ins . F ig . 11 presen t s m e c h a n i c a l p r o p e r t i e s of th ree s teel of s imi la r c o m -pos i t ion vvhich vvere ro l l ed u n d e r c o n d i t i o n s of con t ro l l ed recrys ta l l iza t ion . In b o t h m i c r o a l l o y e d s tee ls m u c h be t te r p rope r t i e s are a c h i e v e d than in the c o m p a r a t i v e s teel dovvn to abou t 800° C w h e n the t r a n s f o r m a t i o n of aus t en i t e du r ing the rol l ing, the d e f o r m a t i o n h a r d e n i n g , and the f o r m a t i o n of t ex ture du r ing ro l l ing start to occur . B y the s a m e ro l l ing cond i t i ons the m i c r o s t r u c t u r e of v a n a d i u m s teel is m o r e h o -m o g e n e o u s b e c a u s e of less of m i c r o s t r u c t u r a l u n h o m o g e n e -ity due to the o r i n c o m p l e t e d i n t e rpa s s rec rys ta l l i za t ion . At stili lovver ro l l ing t e m p e r a t u r e s the d e f o r m a t i o n h a r d e n i n g ,

2 0 m m pla te f rom s tee l w i t h 0,18 C ; 0.4 Si , U M n ; 0.02P , 0 .025AI , 0.0042Nb 0,06V, 0.12Cr , 0,21 Cu , 0,10Ni, normal ised y ie ld stress 4 8 8 N / m m 2 , g r a i n size ASTM number l !

5 0 0

VC

NbC

Al N

Cu , Cr, Ni. P. Si

p r e c i p i t a t i o n h a r d e n i n g

d i m i n u t i o n of

g r a i n s ize

s u b s t i t u t i o n

h a r d e n i n g

10 7.

9

K

9 5

24

C - p e r l i t e " C and N interst i t la f harden ing n

N a t u r a l y ield s t ress and y uncon t ro l l ed irnpurit ies

F igure 10. Consti tution of yield stress in a 20 m m sheet of normalized microalloyed steel with 0 .18% C, 0 .4% Si, 1.4% Mn,

0 .025% Al, 0 .042% Nb, 0 .06% V, 0 .12% Cr, 0.21% Cu, and 0 .10% Ni.

Sl ika 10. Zgradba meje plastičnosti v 20 m m pločevini iz normaliziranega mikrolegiranega jekla z 0 .18% C, 0 .4% Si, 1.4% Mn. 0 .025% Al, 0 .042% Nb, 0 .06% V. 0 .12% Cr, 0.21% Cu in 0.10%- Ni.

micros t ruc tura l nonhomogene i t i e s , and strain anisotropy in-crease , therefore a too low finishing temperature has a harm-ful e f fec t . An even h igher strength can be achieved by a p roper coi l ing tempera ture since slow cool ing enables a greater prec ip i ta t ion hardening.

6 E c o n o m y of microa l loy ing

Microa l loy ing is the m o r e eff icient the more dispersoide is d i sso lved at hea t ing before hot vvorking. The quanti ty of d issolved d isperso ide is the greater the closer are the concen t ra t ions of the const i tut ing e lements to the stoichio-met r ic ratio; e.g. the amount of d issolved A1N in austenite will be the greatest if the weight contents of a lumin ium and n i t rogen in steel are 2 : 1. A too great deviat ion of o n e or the o ther e lement causes that less dispersoide is dis-so lved in austeni te , and less precipi ta tes are fo rmed during the rol l ing and the cool ing. This expla ins why at high alu-m i n i u m contents , over 0 .04%, austenite grains are coarser and less s table than at a lower content of a luminium and at the s a m e content of n i t rogen about 0.01 %.

For the stabili ty of austenite grains the nature of pre-c ipi ta tes is not important , only their number and stability mat ter , or more correctly, the number of precipitates per unity of v o l u m e of austeni te . Theoret ical ly the presence of about 0 . 0 3 % A1N or of a cor responding quanti ty of other d i sperso ides of precipi ta tes of equal size is needed to at-tain a suff ic ient stabili ty of austenite grains. The contents of microa l loy ing e lements and of d ispersoide phases giving equal v o l u m e densi t ies of precipi tates are given in Table 1 for the mos t f requen t dispersoides . Alumin ium assures a suff ic ient densi ty of precipi ta tes already at the lovvest con-tent, whi le the highest content is required in the čase of v a n a d i u m carb ide s ince this carbide is much more soluble

E E

cn c o; i i/i a>

10 c <u

C a

i/i i/i

"O

O)

1 0 0 0

5 0 0

5 0 0

1 ! 0 13 C, 0 20 Si. 123 Mn, 0032AI 003Mb

•

T MP / ; ^^ /

•

I o

IK

iR

O cr

100 C o

LU

9 0 0 8 0 0 7 0 0

Finishing temperature, ° C

Figure 11. Influence of fina! rolling temperature on the properties of three steel (Ref.-6).

Slika 11. Vpliv temperature na koncu val janja na lastnosti treh jekel. Po viru2 6 .

in austenite than other phases . Fig. 4 s h o w s the influence of vanad ium and n iob ium in a čase ha rden ing C r M n steel of the same compos i t ion , and vvithout a lumin ium. o n the size of austenite grains af ter annea l ing at the temperature of 920° C 1 6 . The equal e f f ic iency in h inder ing the austenite grain grovvth is achieved by an about 5 t ime smal ler content of n iobium. The reason is the previous ly men t ioned higher solubili ty of vanad ium carboni t r ide in austeni te at the nor-

mal iz ing tempera ture . T h e better ef f ic iency of n iob ium in the control of the austeni te grain size indicates that it is a more e c o n o m i c microa l loy ing e lement . Its deficiency is a lower precipi ta t ion hardening dur ing the cooling af ter the normal iz ing , thus the proper t ies of steels are intproved onIy because of grain refining. In order to es t imate the econonty of mic roa l loy ing it is necessary to knovv also the interact-ing e f fec t s of d isperso ide phases , i.e. the capabil i ty of an element to dis integrate the d i sperso ide phase of other ele-ments , the reactivi ty of microa l loy ing e lements with other e lements in steel vvhich do not fo rm efficient dispersoide phases , the d i f fus iv i ty of microal loying elements , and their min imal eff ic ient conten ts . Data on the free fomia t ion ener-gies of d isperso ide and on the di f fus ivi t ies of microal loying e lements in aus teni te are gathered in Table 2. The higher is energy of f o m i a t i o n the greater is the stability of the dispersoide phase .

Table 1.

Element Dispersoide A, % B, %

Al 0 .01 Al 0 .015 at 0 .008 N A1N N b 0 .024 N b 0 .036 at 0 .18 C N b C Ti 0 .015 Ti 0 .022 at 0 .18 N TiC V 0 .018 V 0 .039 at 0 .02 N VN

V 0 .15 at 0 .18 C v c V 0 .05 at 0.5 C v c

A—Content of microalloying element vvhich ensures an equal volume of precipitates, and B—Minimal content of microalloying element vvhich hinders the grovvth of austenite grains at 920°C.

Table 2. Free energies of fomiation of dispersoide and the diffusivit ies of constitutive metals

Free ene rgy of fomia t i on Diffus ivi ty , c m 2 / s kca l /mole kca l /mole at 920° C

A1N 46 .7 Al 7.0 x 1 0 - y

TiN 53 .5 T iC 4 0 Ti 7 .3 x 1 0 - 1 2

V N 17.5 V C 18 N b N 32 N b C Nb 8 .85 x 1 0 - 1 2

The mos t s table c o m p o u n d is t i tanium nitride. At si-ntultaneous p re sence of several microal loying elements , ni-trogen vvill in equ i l ib r ium condi t ions first of ali bind to tita-nium, probably a l ready in the melt , dur ing the solidification, or soon a f te r the sol idif icat ion of steel. Ti tanium consumed , nitrogen can b ind to a lumin ium, then to n iobium, and finally to vanadiunt . Th i s o c c u r s only at annealing vvhich ensures the equi l ibr ium. By shor ter anneal ing, also at nomial iz ing, also the rate of prec ip i ta t ion rnust be considered. The latter is the higher the h igher is d i f fus iv i ty of cat ions in austenite (nitrogen and ca rbon are interst i t ials and their diffusivi ty is for several o rders of m a g n i t u d e greater, therefore their diffusion can be neglec ted vvhen c o m p a r e d vvith that of alu-minium, t i tanium, etc.) . The re fo re the highest fomia t ion rate vvould be expec ted for A1N, than by VN, N b N , and fi-n a l i TiN. T h e rate of precipi ta te fomia t ion depends also on the content of microa l loy ing e l emen t s in the ne ighbourhood of precipitation nuelea . The greater is the content expressed in atom.% the h igher is its amoun t in precipi tate though its

d i f fusivi ty is lovver than the d i f fus iv i ty of e lement vvith a smaller content in steel.

Aniong the carb ides the mos t s table is t i tan ium carbide, follovved by n iobium, and finally v a n a d i u m carbide vvhich is comple te ly dissolved in austeni te a l ready at the normal iz -ing temperature. Also the solubil i ty of v a n a d i u m nitr ide is relatively high. Thus v a n a d i u m is eff ic ient as microa l loying e lement for the control of austeni te grain size only at con-centrat ions above 0 .1%, vvhile at lovver conten ts it p roduces a precipitat ion hardening if sueh ha rden ing is poss ib le due to the thermal regime.

Microal loying e lements react also vvith o ther e l ements and fo rm compounds vvhich do not ha rden the steel nei ther influence the mobil i ty of austeni te grain boundar ies , thus they have no inf luence on the grain size. E.g. a lumin ium is first bound to oxygen and only then to n i t rogen. Similar is the behaviour of t i tanium vvhich reacts addi t ional ly also to sulphur. N iob ium and v a n a d i u m react in normal ly de-oxidized steel only to n i t rogen and ca rbon . The re fo re the microal loying vvith a lumin ium and t i tan ium is eff ic ient only if care is taken that they are not c o n s u m e d in react ions to oxygen or oxygen and sulphur.

Consider ing ali the influential parameters , the microa l -loying vvith a lumin ium is the least expens ive . In steel m a n -ufac tured by mel t ing in electric are f u m a c e o n e ftnds habi t-ually 0 .008 to 0 . 0 1 2 % ni t rogen and thus vvith 0 .02% alu-min ium vvhich is not bound to oxygen a suff ic ient stability of austenite grains is achieved if the annea l ing tempera-ture does not exceed 9 2 0 ° C . For addi t ional sa fe ty a higher content of a lumin ium 0 .025 to 0 . 0 3 0 % is r e c o m m e n d e d . Greater amount does not increase the e f fec t of microa l loy-ing since the solubil i ty of A1N in austeni te is d imin ished and a higher temperature of hea t ing of the steel be fo re rolling is required. This is not economica l and it is o f t en also ha rmfu l for the steel proper t ies af ter the rol l ing. A higher a lumin ium content is also not requi red for deoxida t ion since already 0 .01% Al reduces the solubil i ty of o x y g e n in steel melt to about 5 p p m 2 7 . A l u m i n i u m does not bind to car-bon and sulphur therefore its e f f ic iency d o e s not depend on the amount of those tvvo e lements in steel. Data on the A1N solubili ty in austeni te conta in ing m e d i u m and high concentra t ions of ca rbon are not d isponible , therefore it is not knovvn vvhether the solubili ty p roduc t in those steels is equal to that in steel vvith up to 0 .2% C. Also if the solubil-ity product vvould be the same, a d i f ferent e f fec t of the same quant i ty of precipi tates as in the lovver ca rbon steel is to be expected. The reason is that the mobi l i ty of austenite grain boundar ies depends also on the sur face energy of austenite vvhich fur ther depends on the quant i ty of a l loying e lements and impuri t ies in solution.

Ti tanium diminishes the e f fec t of A1N since it could desintegrate this phase at longer annea l ing , e.g. dur ing the čase hardening anneal ing. T i tan ium dec reases also the e f -fect of n iob ium and vanad ium. T h e r e f o r e the combina t ion of t i tanium and other microa l loy ing e l emen t s does not g ive an effect proport ional to the level of a l loying. Also the combina t ion of a lumin ium and v a n a d i u m is not especial ly efficient at a normal ni t rogen content , it is successfu l only by a ni trogen content of above 0 . 0 1 5 % and a v a n a d i u m con-tents over 0 .1%. This is used in m o d e m steel for thermo-mechanica l (control led) die forg ing of parts fo r car motors . Forgings m a d e of those steel do not require the heat treat-ment after forging because the cont ro l led cool ing ensures a suitable micros t ructure of steel and the required combina-tion of strength and toughness . This steel conta ins also 0.01 to 0 .02% Ti vvith the a im that TiN precipi ta tes fo rmed at the

cool ing after the solidification (Fig. 12), hinder an exces-sive austenite grain growth during the heating before the die forging. In order to achieve an efficient complex microal-loying of such steel vvith aluminium, nitrogen, vanadium, and t i tanium it must be continuously čast into billets vvith a cross sect ions up to 150 x 150 m m vvith magnetic stirring.

Therefore it is evident that the most suitable combina-tion of microal loying elements in standard steel vvith normal nitrogen is a luminium, niobium, and vanadium. The first is bound to nitride, the second into carbonitride vvith about 9 0 % of carbide, therefore they do not interfere mutually. Both are efficient during the rolling, vvhile vanadium con-tributes for the precipitation hardening during the cooling f rom the roll ing or normalizing temperature. Niobium also does not react vvith oxygen and sulphur therefore it is ali available for the improvement of steel properties.

ti

• C" MJ*Š8»» :vl

" % " ' * * *

•.••v-Figure 12. T iN precipitates in steel vvith 0.32% C, 0.02% Ti , 0.015% N, 0 .12% V, 0 .025% Al, 0 .60% Si, and 1.4% Mn. Carbon extraction replica vvas prepared f rom a die forged shaft. TiN precipitates vvere

fo rmed during the cooling belovv the solidification temperature of steel.

Slika 12. Izločki TiN v jeklu z 0 .32% C, 0 .02% Ti, 0 .015% N, 0 .12% V, 0 .025% Al, 0 .60% Si in 1.4% Mn. Ogljena ekstrakcijska replika je bila pr ipravl jena na u topno skovani ojnici. Izločki TiN so

nastali pri ohla janju kmalu pod temperaturo strjenja jekla.

Titanium also reacts vvith oxygen and sulphur, therefore an economic microal loying required a good preceding de-oxidat ion and desulphurisat ion of the steel, i.e. oxygen and sulphur must be bound to a luminium and calcium before t i tanium is alloyed. The solubility of t i tanium nitride in mol ten steel is small , and TiN is formed in the melt already by 0 .02% Ti and 0 .015% N. Titanium nitride formed in the mel t on during the solidification is useless or even harm-ful . Recent re ferences 2 8 r ecommend the titanium contents in steel for controlled forging to be close to 0 .01% vvhich ensures that more nitrogen is free for binding into VN pre-cipitates belovv the solidification temperature vvhich hinder the grain grovvth at heating before the forging.

In general for structural steels the aluminium/niobium/ vanad ium combinat ion is the most efficient and the majori ty of microal loyed steel is based on. The steels for controlled forging and some steels for strips represent an exception. Additional ef lec ts are due to the fact that niobium strongly hinders the inteipass static recrystallization of austenite al-ready at the level of microalloying. Titanium is used in some hot rolled strips, steels for controlled forging, and čase hardening steels vvith higher grain stability and a narrovv-

band hardenability microal loyed also vvith boron. The role of t i tanium in these steels is not the control of austenite grain size or precipitation hardening but the prevention of the binding of boron to ni t rogen vvhich is a precondit ion for the strong boron effect on steel hardenability.

7 Summary

The paper gives a short revievv on the influence of alu-minium, niobium, t i tanium, vanad ium and nitrogen as mi-croalloying elements on the properties of steel. The size and stability of austenite grains, dispersoide phases formed f rom microalloying elements , precipitat ion hardening, in-fluence of microalloying e lements on the hot deformat ion, and the economy use of various elements for microalloying of steel are discussed.

8 References 1 C. Zener: loc. cit. ref. 2. 2 T. Gladman and F.B. Pickering: Journal of ISI 205, 1967,

653. 3 J.E. Bruke and D. Turnbull: Progr. Met. Phzs. 3. 1952,

220. Loc. Cit.: H.V. Atkinskon: Acta Metallurgica 36, 1988, 469.

4 C. Strassburger:- Entvvicklungen zur Festigkeitsteigerung der Stahle, Verlag Stahl Eisen. Dusseldorf. 1976.

5 W.E. Duckvvorth: "Metallurgy of structural steels, present and future possibilities" v: Strong and Tough Steels, ISI STP 104, 1967, 61.

6 D. Drobnjak: Fizička Metallurgija. TH Fakultet. Beograd. 1981, p. 173.

7 F. Vodopivec, M.Gabrovšek in B. Ralič: Metals Science 9, 1974, 324.

8 C.Z. VVagner: Z. Elektrochem. 65.1961. 65. Loc. Cit. F.B. Pickering: "Titanium Nitride Technology" in "Microal-loyed vanadium Steels": Ed. Akad. Gorn. Hutn. Krakov. 1990, 79.

9 F. Vodopivec, M. Gabrovšek, M. Kmetic in A. Rodič: Metals Technology 11, 1984, 481.

10 T. Gladman, D. Dulieu in I.D. Mclvor: "Structure -Properties Relationships in High Strength Microalloyed Steels" in Micro-Alloying 75, UEC. Nevv York. 1977, 32. 11 K. Narita: Trans. ISJ 15, 1975, 145.

12 S. Matsuda in N. Okamoto: Trans. ISJ 18. 1978. 198. 13 F. Vodopivec: Metals Technology 1978, 118. 14 M.J. Grooks, A.J. Garrath-Reed, J.B. Vander Sandt in

V.S.Ovven: Metallurgical Transactions 12A, 1981. 1999. 1 5 B. Dutta in A.C. Sellars: Materials Science and Technol-

ogy 2, 1986, 146. 16 M. Korchinsky; private communication. 17 E. Hornbogen: "Verfestigungsmechanismen in Stahlen"

v Die Verfestigung von Stahl: Climax HB. 1969. 1. 18 E. Hornbogen in E. Minuth: Arch. Eisenh. 44. 1973. 621. 19 R.B.G. Jeo, A.G. Melville, P.E. Repas in J.M. Gray: J.

Metals 20. 1968. 33. 2 0 F. Vodopivec in M. Gabrovšek: Železarski zbornik 21.

1987, 19. 2 1 J.J. Jonas in I. Weiss: Metal Science 3, 1979. 238. 22 I. Weiss in J.J. Jonas: Metallurgical Transactions 11 A.

1980, 403. 2 3 F. Vodopivec, M. Kmetič in A. Rodič: Železarski zbornik

18, 1984, 9. 24 A. le Bon, J. Rofes-Vernis in C. Rossard: Metals Science

9, 1975, 36. 2 5 D. Kmetič. F. Vodopivec in M. Gabrovšek: Železarski

zbornik 14, 1980. 39. 26 F. Vodopivec, M. Gabrovšek, J. Zvokelj in M. Kmetič:

Metallurgical Science and Technology 8, 1991, 103. 27 E.T.T. Turkdogan: Arh. Eisenh 54. 1983. 1 2 8 T. Shiraga, K. Matsuinoto, S. Suzuki. M. Ichiguro. H.

Kido in T. Abe: NKK Techntcal Revievv 53. 1988. 1.

The Dependence of the Heat Energy Consumption upon the VVorking lntensity and the Frequency of the Isolation Maintenance of a Pusher-type Furnace

Ovisnost utroška toplinske energije od intenziteta rada i učestalosti održavanja izolacije potisne peči

J. Črnko, Metalurški fakultet Sisak, Aleja narodnih heroja 3, 44103 Sisak

The paper covers investigations of the specific heat energy consumption dependence upon the productivity and frequency of the isolation maintenance in a pusher-type furnace of a strip and billet rolling mili.

U okviru ovog rada istraživana je ovisnost specifičnog utroška toplinske energije od produktivnosti i učestalosti odražavanja izolacije na primjeru potisne peči u valjaonici traka i gredica.

1 Introduction

Systematic decreasing of the fuel consumpt ion per unit of a product should be one of the most important tendencies in Yugoslav rolling mills. However , the dynamics of the spe-cific fuel consumpt ion decreasing has recently been stopped in some heat ing furnaces and has assumed, for the various reasons (mostly objective), an inversed trend. Thus, even a decreased vvorking intensity of heating furnaces leads to an inereased specific fuel consumption. This often happens to appear in the rolling mills ment ioned, especially lately. This is the reason that the paper deals with the investigation of the specific heat energy consumption dependence upon the productivi ty of heating furnaces and, especially, upon the f requency of isolation maintenance on the example of one of the two pusher- type furnaces of a Sisak Ironwork 's strip and billet rolling mili (Sisak, Croatia).

2 Heat energv balance and fuel consumption

Rough values for specific fuel consumption decreasing in the čase of inereased working intensity of heating fur-naces can be obta ined by heat energy ba lance 1 ' 2 . The total heat energy consumpt ion ( ]T E , ) can be derived into two groups:

1. one depending on the mass of steel being heated in a certain heat ing furnace:

• heat energy of steel being heated (Ei),

• heat energy of scrap developing in the course of steel heating ( E 2 ) ,

• heat energy losses ( through the walls to the out-side, by eool ing water etc.) independently to the mass of steel being heated ( £ 3 ) ;

2. one depending on the level of fuel utilization in a cer-tain heating furnace:

• heat energy losses because of mechanical un-burning, incomplete chemieal combust ion and by fuel gases going out ( E 4 ) .

Obviously, it is (per unit share)

Ex + E2 + E3 + E4 = l ( 1 )

First, let 's take that a quantity of condit ional fuel A „ is consumed to heat a mass of steel Mn, i.e.

E1Xn + Eo Xn + E3 Xn + E4 Xn = Xn (2)

If the intensity of steel heat ing inereases 2 t imes, the consumption of the condit ional fuel inereases up to the value of A ' 2 , and the equation (2) takes the follovving form:

(3) z(L\ + E2) Xn + E3Xn + E4Xt = Xz

From the equation (3) we can now derive a quantity of the conditional fuel X , , i.e.

A 2 — A , z{Ex + E2) + E3

1 - E4 (4)

Based on these, it is possible to define a decrease of the specific fuel consumption, as a result of the intensity of steel heating inereased r times, as follovvs:

Ax = A, AL

A .

zMr,

• 1 0 0 =

• 100 =

A , + B1) + E3\

1 -

zMn

z{El+E2) + E*

z{ 1 - E4)

100

100 (5)

Consequently, as a result of the inereased vvorking inten-sity of the heating furnaces, i.e. the process of steel heating being intensified ; times, the the specific fuel consumption (in %) vvill be decreased in a ratio:

z(Ex + E2) + E3

r ( l " E4) • 100. (6)

3 M a i n character is t ics of a pusher-type furnace

A three-zonal pusher- type fu rnace observed, des igned by Amer i can Rust Furnace Co., vvas built in the year 1972. Beg inn ing f r o m the charging side the f u m a c e has a pre-hea t ing zone , a hea t ing zone and a soaking zone. In the prehea t ing and heat ing zone a charge vvas heated both f rom the upper and the lovves side, vvhereas in the zone of soaking it was hea ted only f r o m the upper side. Burners vvere in-stal led laterally on the furnace , above and belovv the charge in the prehea t ing and heat ing zone. In the preheat ing zone five burners vvere instal led above the charge and five burn-ers belovv the charge . Tvvo burners vvere installed above the charge on one side and three on the other side of the fu rnace . T h e burners vvere also installed belovv the charge, but p laced inversely to those above the charge. In the heat-ing zone six burners vvere installed above the charge and six burners belovv the charge. On each side of the furnace three burners vvere p laced above the charge and three belovv the charge . In the soaking zone six burners vvere placed on the f ronta l side of the f u m a c e . Othervvise, the furnace is not of a convent iona l profi le and is of a convent ional temperature r e g i m e 3 . By opera t ing measu remen t s it vvas found that the firing f r o m the lovver side and that f rom the upper side have the equal eff ic iency. The fu rnace profile, as vvell as its ma in d in tens ions are presented schematical ly in Fig. 1.

H 1 1 I § 1 1 I U I I 1 ffl ITI

-I F igu re 1. S c h e m a t i c presenta t ion of a pusher- type fu rnace profi le .

Sl ika 1. Shemal sk i pr ikaz profi la pot isne peči.

During the energet ic balancing slabs of St 12 (per DIN) quality, d in tens ions of 4 3 0 x 190 x 3 8 0 0 m m and mass of 2 5 0 0 kg s ingly vvere heated in the pusher- type furnace. The f u r n a c e vvas fired by natural gas having the heating value of 37300 k J / m 3 . Gas consumpt ion vvas 2347 m 3 / h , and consumpt ion of air necessary for its combus t ion vvas 24286 m 3 / h . Air t empera ture at the metal recuperator exit vvas 3 0 0 ° C . Accord ing to the request of the rolling mili train the fu rnace product iv i ty vvas kept at 37 t/h, and the slabs vvere hea ted up to the final tempera ture of 1230 -1250° C. Hovv-ever, the fu rnace vvas des igned to achieve the product ivi ty of 67 t/h vvhen heat ing slabs of stated quali ty and dinten-s ions . Th i s shovvs that the coeff ic ient of capaci ty util ization of the f u r n a c e vvas about 0 .55, vvhich indicates that vvorking intensi ty of the fu rnace can be increased 1.81 times.

4 Fuel c o n s u m p t i o n dependence upon the productivity and frequency of a pusher- type furnace maintenance

For a ca lcula t ion of the specif ic fuel consumpt ion decrease heat energy consumpt ion in a pusher- type furnace vvas found .The heat energy consumpt ion (per single i tems) in

the furnace vvhen heat ing the slabs of the stated qual i ty and d imens ions is given in Table 1.

Table 1. Heat ene rgy c o n s u m p t i o n in a pushe r - type f u r n a c e vvhen heat ing slabs of St 12 qual i ty and d imens ions of

4 3 0 X 1 9 0 x 3 8 0 0 m m

Heat energy consumpt ion i tems

Quant i ty of heat energy Heat energy consumpt ion i tems M W %

Ei 8.998 33.32

E-, - -

e3 6.108 22.61

EA 11.902 44 .07

LE, 27 .008 100.00

The follovving step enables to f ind out hovv m u c h the specific fuel consumpt ion decreases if the vvorking intensity of the furnace increases 1.81 t imes.

Applying the equat ion (6) we get that the specific fuel consumpt ion decreases for 18 .09%, providing that, as stated, the vvorking intensity of the pusher - type furnace in-creases 1.81 t imes, i.e.

1 .81 0 . 3 3 3 2 + 0 . 2 2 6 1 \

1 .81(1 - 0 . 4 4 0 7 ) J 100 = 18.099?

The value obta ined also anables calcula t ing the specific fuel consumpt ion in the čase that the fu rnace vvorks full capacity. Resul ts of such a ca lcula t ion shovv that the specific natural gas consumpt ion in the fu rnace decreases f r o m 63.43 to 51.96 m 3 / t , cor responding to the decrease of the specific heat energy consumpt ion f r o m 2366 to 1938 kJ/kg, i.e. for 4 2 8 kJ/kg.

To get a real picture of the d e p e n d e n c e of the specific heat energy consumpt ion upon the vvorking intensity of the puscher- type furnace , a three-years per iod of the furnace vvork has been analyzed. Only the data referr ing to the furnace vvork during the St 12 qual i ty slabs of 4 3 0 x 190 x 3 8 0 0 m m dimens ions being hea ted vvere delt vvith. The dependences of the specific heat energy consumpt ion upon the furnace product ivi ty be ing got that way are presented in a fo rm of a d iag ram in F ig . 2.

3500

3000

2500

. 2000

o 1500

1000L-30 35

_ j i I I i 0 45 50 55 50 65

Produc t i v i t y , t / h

Volume in m 3 re fers to a nomia l state.

F igu re 2. The d e p e n d e n c e of the spec i f ic heat ene rgy consumpt ion upon the pusher - type f u m a c e product iv i ty .

Sl ika 2. Ovisnos t speci f ičnog u t roška top l inske energ i je od produkt ivnos t i po t i sne peči .

J. Črnko: Ovisnost utroška toplinske energije od inteziteta rada i učestalosti održavanja izolacije potisne peči

C o m p a r i n g the calcula ted va lues for the specific heat energy c o n s u m p t i o n with those obta ined by operat ing mea-surements for the stated qual i ty and d imens ions of the charge, the results of vvhich are presented in a fo rm of a d iag ram in Fig. 2, we can see that the d i f fe rences are rela-t i v e ^ small . Cons iderab le d i f fe rences are the consequence of inc luding a consumpt ion of natural gas for "b l ank" fir-ing to the opera t ing data o n the natural gas consumpt ion. Also, the changes of energet ic losses per i tems of energetic balance , in the course of the fu rnace utilization for its iso-lation repara t ion betvveen the two stoppings, can bring to some b igger d i f f e rences betvveen the calculated and oper-ating va lues for the specif ic heat energy consumpt ion in a certain fu rnace product ivi ty .

2500

0 2 4 6

Maintence frejuence, mth Figure 3. The dependence of ihe specific heat energy consumption upon the frequency of the isolation maintenance and the pusher-type

furnace floor cleaning. Sl ika 3. Ovisnost specifičnog utroška toplinske energije od učestalosti održavanja izolacije i čiščenja poda potisne peči.

The analys is carr ied out for the fu rnace vvork betvveen its last tvvo s toppings , because of the beam cleaning and of the isola t ion repara t ion , shovved that vvith the produc-tivity of 50 t/h (real ized annual product ivi ty) the specific heat energy c o n s u m p t i o n increases f rom the initial 1480 to the final 2 2 4 0 kJ /kg , in o ther vvords the average value is 1860 kJ/kg (Fig . 3). Bes ide others , the course of that is al-most up to 8 0 % of re f rac tory m a s s falls off the skid carrier so that energy losses by cool ing vvater are cons iderab le 4 . The increase of isolat ion main tenance f requency, as vvell as that of b e a m c lean ing f r o m the layers, to the half of tirne (6 months ) of the p rev ious (12 months ) vvould decrease the

average heat energy consumpt ion f r o m 1860 to 1670 kJ/kg (Fig. 3), i.e. for 190 kJ/kg.

Hovvever, for a rather long per iod of t ime in s o m e vvest-ern countr ies a quartal f r equency of pusher - type f u m a c e s 5

maintenance has been pract ised, so regard ing to this, there are no reasons to m a k e the first step in our strip and billet rolling mills as vvell.

5 S u m m a r y

Results obtained analytical ly shovv that the specif ic heat en-ergy consumpt ion decreases for about 18% if the p roduc-tivity of the pusher- type fu rnace obse rved increases f r o m 37 to 67 t/h. Also, the opera t ing data analysis of the fur-nace vvork does not shovv s ignif icant devia t ions f r o m the results obtained analytically. Hovvever, because of the lack of coordinat ion betvveen pusher- type fu rnaces and roll ing mili train capacit ies , even in normal p roduc t ion condi t ions it is impossible to realize. T h e s a m e way, the increase of the pusher- type fu rnace isolat ion ma in t enance f requency and that of the layers removal f r o m the floor to the per iod of 6 f rom so far 12 months , vvould decrease the specif ic heat energy consumpt ion for about 10%. Th i s can be real ized successful ly by better m o n t h and vveek p lann ing of rol l ing, vvhich vvould assure heat ing of the charge hav ing the same quali ty and d imens ions in the cour se of a fevv vveeks. In such a čase it could be poss ib le to s top o n e of the fu rnaces if ano ther ' s passing capaci ty per hour vvould sa t is fy the re-ques ts of the roll ing mili train. Such per iods some t imes occure now, too, as vvell as those in vvhich p lans of rolling change f rom shift to shift together vvith p lans of heat ing.

6 References 1 W. Heiligenstaedt: Warmetechnische Rechnungen fiir In-

dustrieofen, Verlag Stahleisen M.B.H., Dusseldorf, 1956, s 6 - 1 3

2 A.N. Nesenčuk et al.: Ognetehničeskie ustanovki i toplivosnabženie, Izdatel 'stvo "Vyšejšaja škola", Minsk, 1982, s 35^46

3 V.A. Krivandin, JU.R Filimonov: Teorija t kon-strukcij metallurgičeskih pečej, Izdatel 'stvo "Metallurgija", Moskva. 1986. s 360

4 Ž. Acs, Lj. Milič, M. Kundak: Zbornik IV. savetovanja "Energetika i zaštita čovjekove sredine u crnoj metalur-giji", UJŽ Beograd, Herceg Novi, 1985, s 119-125

6 L.J. Grafe: Improving energy utilization in steel reheat furnaces, Iron and Steel Engineer, Vol. 62, No 1, 1985, s 43 -*7

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Tool-steel Wire Dravving at Elevated Temperatures

Vlečenje žice iz orodnih jekel pri povišanih temperaturah

B. Arzenšek, B. Šuštaršič, G. Velikajne, Inštitut za kovinske materiale in tehnologije, Ljubljana I. Kos, K. Zalesnik, F. Marolt, Železarna Ravne, Ravne na Koroškem

Tool steels have in cold state very low workability, therefore they must be frequently recrystallization annealed during the cold drawing proeess, but some types of steels cannot be cold dravvn at aH. Their working properties are highly improved at elevated temperatures. This paper analyzes the drawability of wire, made of BRM2 (VV.NR-1.3343) steel, and it deseribes the technology of wire dravving at temperatures up to 700' C.

Orodna jekla imajo v hladnem stanju zelo slabe preoblikovalne sposobnosti, zato jih moramo med hladnim vlečenjem velikokrat rekristalizacijsko zariti, nekatera jekla pa hladnega vlečenja sploh ne prenesejo. Njihove preoblikovalne sposobnosti se precej izboljšajo pri povišanih temperaturah. V delu smo ugotavljali vlečne sposobnosti žice iz jekla BRM2 (VV.NR-1.3343) in opisali tehnologijo vlečenja žice pri temperaturah do 70(f C.

1 Introduct ion

High-speed tool steel of B R M 2 type is main ly used for tools and spiral drills for mach in ing of steels and alloys, and for manufac tu r ing of high-qual i ty wear-resistant tools. Wear resistance of steel is achieved by a great number of line secondary carb ides vvhich are in the ferrit ic matrix of steel. Carbides do not give only a high vvear resis tance of steel but they also cont r ibute to an intensive hardening of steel in cold workabil i ty. T h u s the vvire made of B R M 2 steel can sustain in cold dravving a lmost 2 0 % partial and approximately 4 5 % total r edue t ion degree , therefore it must be more than six times in te rmedia te ly recrystal l izat ion annealed in dravving from 8 to 2 m m diameter . Such a manufac tu r ing of fine vvire is ra ther expens ive and long lasting.

Drawabi l i ty of steel is rather improved at e levated tem-peratures w h c n vvire can be dravvn to fine d imens ions vvith-out in termedia te recrystal l izat ion anneal ing. The first dravv-ing tests at e levated tempera tures (vvorm dravving) vvere made vvith shor te r lengths of vvires, and it vvas found that vvire can be dravvn at 7 0 0 ° C even to the d iameter of 3 m m . Based on resul ts of dravving tests to thin d imens ion vvires, vvhich vvill be deser ibed in details, the technology for vvorm dravving of vvires in coils vvas prepared.

2 Drawabi l i ty of the steel at elevated temperatures

Dravving tests of thin vvires shovved that drawabil i ty of BRM2 steel is op t imal at 7 0 0 ° C and 15% reduetion de-gree. Drawabi l i ty of the steel vvas de termined f rom num-bers of dravvs vvhich vvire susta ined at var ious temperatures and f rom yield s t resses calcula ted by follovving mathemat -ical express ion:

- J L q , n ~ A A i '

vvhere is:

qm yield stress in N / m m 2 , F, dravving force in N / m n i 2 and A A i eross-see t ion reduet ion in single dravv in

m m " .

Yield stresses and number of dravvs, vvhich vvire of B R M 2 steel sustained at var ious dravving tempera tures , are presented in Fig. 1. The plot shovvs that the vvire sustains

2600

2 4 0 0

% 2000 E z ~ 1600 cr

S 1200 in

V 800

4 0 0

0 (

Log d e f o r m a t i o n ip — i 1 i l i l l l

8,28 7,58 6.43 5.46 4.63 3.93 3.35 2,80

D i a m e t e r of vvire ( m m ) »-

Figure 1. Yield stresses at vvire dravving of B R M 2 steel at elevated temperatures.

Slika 1. Preoblikovalne napetosti pri v lečenju žice iz jekla BRM2 pri različnih temperaturah.

the cold-dravving reduet ion vvithout recrystal l izat ion anneal-ing f rom 8 to 5.5 m m . It can sustain s o m e more dravvs at tempera tures up to 5 5 0 ° C , vvhile at 7 0 0 ° C it can be reduced even to 3 m m . G o o d workabi l i ty of the steel can be judged

o 7 0 0 ° C « K X A - L a s t poss ib l e d r a w • 6 0 0 ° C of w i r e A 5 0 0 C • 3 0 0 ° C a 2 0 ° C

/ / /

/ \ / _ • tt

V

- i Draw of

1

£ = 15 7 . - i

Draw of 1

wire : 3 5 7 9 11

0.4 0,8 1, 2 1, 6 2 0 2.2

also by yield stresses which values at 700°C are half of the values like in cold dravving. In hot drawing of the wire the recrystall ization annealing is not needed since recovery is achieved by heating wire to the drawing temperature and during the dravving process, and at higher temperatures also recrystall ization probably occurs. The described tests were made with 2 to 3 m long vvire pieees on drawing bench with a dravving velocity of 0.25 m/s. Drawing conditions on a dravving bench are less demanding than in coil dravving. Therefore vvire in coil dravving sustains less dravvs.

3 Technologv of vvire dravving at elevated temperatures

The aim in preparing the technology of vvire dravving at elevated temperatures was to apply the existent equipment for cold dravving to the maximal extent. This on one side reduces the developing costs of the technology, and on the other side it enables cheaper and simpler transmission of the technology into industrial practice. The scheme of thus des igned and also tested line is given in Fig. 2, while Fig. 3 shovvs the picture of it.

Heot nq S t ra i gM en ng ; t • . i ••g

Figure 2. Scheme of vvire-dravving line at elevated temperatures.

Slika 2. Shema linije za vlečenje žice pri povišanih temperaturah.

Figure 3 . Picture of vvire-dravving line at elevated temperatures, built at the Institute of Metals and Technologies in Ljubljana.

Slika 3. Izgled linije za vlečenje žice pri povišanih temperaturah, ki je postavl jena na Inštitutu za kovinske materiale in tehnologije v

Ljubljani.

The presented line consists of three basic units:

• equipment for conduct ive heating of vvire,

• equipment for temperature measurements on vvire dur-ing the dravving and

• dravving machine.

In front of the heating equipment also f rame for uncoil-ing and straightening rollers were m o u n t e d In the vvhole installation only the conductive heating equipment for vvire was new. Wire is heated by three pairs of contact rolls. The basic characteristics of the three ment ioned units are:

3.1 Heating Ec/uipment

This equipment vvas purchased at Montanstahl , Svvitzerland. Its power is 90 kVA, and this vvas chosen according to the greatest desired diameter 8 m m of heated vvire, the high-est vvire temperature 800° C, and the dravving velocity 0.28 m/s. Heating of vvire is a two-stage process. In the interval betvveen the first and second pair of rolls the vvire can be heated at most up to 5 0 0 ° C , and betvveen the second and the third pair the temperature is raised further to 800° C. Heating povver is adjusted manual ly vvhile the equipment contains also drivver vvhich prevent overheat ing of vvire in the čase of stoppages during the dravving process.

3.2 Equipment for Measuring Wire Temperature

Wire temperature is measured with optical pyrometer. Mea-suring system is not connected vvith the heating system, therefore the heating povver is regulated manual ly accord-ing to the registrated temperature.

3.3 Draw'ing machine

Dravving machine is designed for cold dravving of vvires thinner than 8 m m . Therefore the dravving d rum is con-structed in such a way that dravvn vvire in cold dravving slips uniformly along the drum. Dravving condit ions are during the cold dravving rather unchanged. In vvorm dravv-ing the conditions are changing according to the temperature of vvorm dravving, vvire diameter, and lubrication. In the first vvorm-dravving tests vvith the described equipment , slippage of vvire on the dravving d rum in the upvvard direction oc-cured, vvhile overlapping of vvire vvindings vvas experienced vvith thinner vvire. Both phenomena made dravving impossi-ble. The described troubles vvere solved by mount ing spe-cial guides for dravvn vvire vvhich vvere fixed around the dravving drum. The ment ioned guides can be dismounted f rom the d rum after vvorm dravving so that the same dravving machine can be used also for cold dravving of vvire.

Efficiency of vvorm dravving of vvire does not depend only on the sliding of vvire along the dravving drum but also on the preparation of vvire surface before dravving, on the quality of lubrication, and thus also on the dravving temperature and the vvear resistance of dies.

3.4 Preparation ofWire Surface

Efficiency of conductive or contact heating depends a great deal on the quality of contacts betvveen the pair of heat-ing rolls and the heating vvire. In bad contacts sparking, incipient melt ing and quenching of heated vvire on contact area can occur. Such an area does not sustain deformation in dravving, and vvire breaks. Sparking can occur on badly descaled or rusted areas of vvire, therefore the vvire surface must be vvell prepared before dravving. According to the experiences obtained so far, the sandblasted vvire surface is the most suitable one especially if it is also copperized. Sandblasted surface is rougher than a pickled one, and thus adhesion of lubricant on the vvire surface before dravving is better. Copperizing on the other hand prevents rusting of vvire after the dravving.

A o te r c o o l i n g

Tempe ra t j re measu remen t

3.5 Lubrication

In w o r m drawing lubrication is less effect ive than in cold drawing, therefore the wire cannot stand higher partial de-fomiat ions degree than 15%. Graphi te was used as lubricant since it was in the regard to persistance and priče the only acceptable lubricant which could stand drawing also at tem-peratures above 500° C. Graphi te has good lubrication prop-erties but its adherence to the wire surface is unfortunately weak, therefore it was applied in form of oil paste. Paste has good lubrication power up to 650°C, at higher tempera-tures its lubrication power is rather reduced due to reduced adherence to the wire surface. Ground and flaky graphite was tested too. A little better lubrication was achieved with flaky graphite but it is rather more expensive. In hot dravv-ing the greatest troubles are caused in lubrication. This problem is not suitably solved, therefore some manufactur-erers of fine sections of special steel have substituted hot drawing with rather more expensive microrolling process, vvhere ali the problems with lubrication are avoided.

3.6 Wear Resistance of Dies

In dravving hard-metal dies are used so far, but they have good wear resistance only up to 300 or 400° C. At higher temperatures their wear resistance is rather reduced, there-fore also ceramic dies, made of silicon carbide, were tested. The ment ioned ceramic material has a very good wear re-sistance even up to 1000°C, but its disadvantages is a low resistance to temperature shocks and its brittleness. In cold and hot drawing tests it was found that ceramic materials stood the dravving process therefore the tests of dravving through ceramic dies vvill be continued.

In developing the technology of hot dravving of vvire a great attention was given also to the yield of dravvn vvire vvhich is lovver than in cold dravving. In hot dravving the beginning and the end of vvire coil must be dravvn cold. Since B R M 2 vvire sustains only tvvo coil dravvs the cold dravvn ends should be cut off . Thus the yield of dravvn vvire is reduced. A trial vvas made to avoid cutting-off the vvire ends by vvelding another material vvith good workability on the vvire end. T h e ment ioned solution proved as unsuitable due to too long needed t imes of sof t annealing the vveld. Therefore the problem vvas solved by annealing the cold

dravvn vvire ends. For this purpose a special conduct ive equipntent for vvire-end annealing vvas built vvhich enabled annealing of 5 m lengths and thus the yield of B R M 2 vvire in hot dravving vvas increased to 100% nearly.

4 Conclusion

Dravving of vvire at elevated temperatures rather differs f rom the cold dravving, therefore many problems appeared in de-velopment the dravving technology at elevated temperatures, vvhich vvere more or less successful ly solved to such an ex-tent that already industrial dravving line for vvire coils exists. So far over 1500 kg vvire of B R M 2 steel vvas dravvn f rom 8 m m to various finer dimensions. T h e finest diameter of dravvn vvire vvas 3,2 m m . The line is improved to such an extent that a large-scale product ion is negotiated vvith the Ravne Iron and Steelvvorks as the tool-steel producer. Dou-ble applicability of dravving equipment , i.e. for cold and hot dravving, the developed technology is much cheaper than the competi t ive microrol l ing, and it is suitable mainly for smaller manufacturers of various tool steels.

5 References J B. Arzenšek, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt,

G. Velikajne: Tehnologija vlečenja jekla BRM2 pri povišanih temperaturah, Poročila inštituta za kovinske ma-teriale in tehnologije v Ljubljani, Nal. št. 91-035, Ljublja-na, 1992

2 K.D. Maraite: Ein Beitrag zur Optimierung des Halbvvar-mziehens, Stahl und Eisen, Umformtechnische Schriften -Band 13, 1988, Verlag Stahleisen, Dusseldorf

3 H. Tzscheutschler: Untersuchungen der Einsatzmoglich-keiten des Halbvvarmziehens, Doktor Dissertation, Aachen, 1982

4 B. Arzenšek, I. Kos, A. Godec: Vlečenje žice iz orodnega jekla Č.7680 - II. del. Poročila Metalurškega inštituta v Ljubljani, nal. št. 85-063, Ljubljana, 1985

5 B. Arzenšek. I. Kos, A. Godec: Vlečenje jekla Č.7680 prt povišanih temperaturah, Poročila Metalurškega inštituta v Ljubljani, Nal. št. 86-037, Ljubljana. 1986

b I. Kos, B. Šuštaršič, B. Arzenšek, V. Leskovšek: Razvoj tehnologije vlečenja pri povišanih temperaturah - II. del, Poročila Metalurškega inštituta v Ljubljani, nal. št. 80-018, Ljubljana. 1990

TEHNIČNE DEJAVNOSTI

ERZ — elektro delavnice, regulacija in zveze

— elektroinstalaci jska dela in popravi la elektro motor jev — popravi la in remonti regulaci jskih sistemov — montaža in popravi la kl imatizaci jskih in tehialnih naprav — sen/is ročnih elektr ičnih in gospodin jsk ih strojev — izdelava in vzdrževanje žičnih, brezžičnih in računalniških

SD — strojne delavnice

— izdelava in popravi lo strojnih rezervnih delov — termična obdelava — izdelava odkovkov do teže 30 kg — izdelava in popravi lo orodi j in hidravl ične ter pnevmatske

opreme — izdelava in komplet i ranje metalurške in sorodne opreme

KD — konstrukcijske delavnice — izdelava, popravi la in montaža jeklenih konstrukci j — metalizacija in navar janje delov z različnimi dodajn imi

material i — popravi la in remonti žerjavov in žer javnih prog

KOVIN — kovinska industrija

— izdelava opreme in elementov za gradbeništvo — izdelava hlevske opreme — izdelava proizvodov galanteri je za trg in kooperaci jo — izdelava delov za manjše individualne naročnike

OD — obrtne delavnice — obrtna kleparska in plast ičarska dela — izvedba sanitarnih, toplotnih, mazalnih in hidravl ičnih ter

ostal ih industr i jskih instalacij — izdelava in montaža lesnih izdelkov za manjše serije in

indiv idualne naročnik

TIDN — tehnične izboljšave delovnih naprav — projekt i ranje de lovn ih naprav (strojno, elektro, hidrav-

l ično in pnevmatsko) — projekt iranje in uvajanje procesno vodenih tehnološk ih

procesov

If* 6 4 2 7 0 J E S E N I C E , C e s t a Ž e l e z a r j e v 8 - t e l e f o n : ( 0 6 4 ) 8 1 - 3 4 1 , 8 1 - 4 4 1 , 8 4 - 2 6 2 -

t e l e f a x : ( 0 6 4 ) 8 3 - 3 9 5 - t e l e x : 3 7 - 2 1 9 , 3 7 - 2 1 2 z e l j s n - t e l e g r a m : Ž e l e z a r n a J e s e n i c e

Resistance of Structural Steel to Crack Formation and Propagation

Odpornost gradbenih jekel proti nastanku in širjenju razpoke

P.D. Odesskij, Centralnij naučnoisledovatelnij institut strojitelnih konstrukcij, im. V.A: Kučerenko, Moskva N. Kudajbergenov, Kazahskoj Himiko—tehnoiogičeskij institut, Čimket, Kazahstan L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana F. Kržič, FAGG, Jamova 4, Ljubljana

Parameters of linear fracture mechanics can be a useful measure for selection of steel with various strength and yield stress (in limits 200 to 1000 MPa). They determine also influence of purity (non-metallic inclusions) and of thermal or thermomechanical treatment. These parameters are effective only if they are measured in the conditions when the plastic deformation at the initial crack growth is limited to minimal value. This happens in corrosion media by measuring K j c and especially in impact loading by measuring I\'fc. These parameters are closely connected with the microstructure and structure of steel. They are suitable for designing structures resistant to brittle fracture if operational (destruction) conditions of those structures are seized, since they occur at high preceeding plastic deformations.

V članku razpravljamo o načinih zboljšanja efektivnih parametrov linearne mehanike loma, ki so merilo kvalitete gradbenih jekel in osnova za izračun konstrukcij odpornih proti krhkemu prelomu. Predmet raziskave je bila skupina maloogljičnih in malolegiranih jekel z napetostjo tečenja 200-1000 MPa. Po kemični sestavi spadajo ta jekla v štiri skupine: maloogljična, mangan silicijeva, manganova mikrolegirana jekla in kompleksno legirana jekla z 0.2-0.6% Mo. Po trdnosti lahko omenjena jekla razdelimo v tri skupine: v skupino z napetostjo tečenja do 290 MPa (normalna trdnost); v skupino z napetostjo tečenja do 390 MPa (povišana trdnost) in v skupino jekel z napetostjo tečenja več kot 390 MPa (visoka trdnost). Lomne karakteristike smo raziskovali pri statičnih in dinamičnih obremenitvah ter v korozijskem mediju. Raziskave smo opravili v skladu z GOST in mednarodnimi standardi, pa tudi po originalni metodiki. Največ smo uporabljali epruvete z ekscentrično obremenitvijo (CTS) (si. 1), dvojno konzolno vpeto klinasto epruveto (si. 2) in cilindrične preizkušance s koncentrično krožno zarezo z utrujenostno razpoko kot koncentratorjem napetosti (si. 3). Pri statičnih obremenitvah smo ugotovili pomembne posebnosti v rasti vrednosti I\1C s trdnostjo jekla takrat, ko je imelo valjano jeklo "racionalno" mikrostrukturo. Ugotovili smo tudi, da raste vrednost I<lc v jeklih s povišano in visoko trdnostjo s čistostjo jekla. Istočasno pa ti parametri niso zelo tesno povezani z mikrostrukturo jekla. Njihova uporaba v inženirskih izračunih pa omogoča oceniti velikost nevarnih napak v konstrukcijah. V članku je tudi pokazano, kako je moč z omejitvijo obsega plastične deformacije (pri dinamičnih preizkusih ali v korozijskih medijih) povečati občutljivost parametrov linearne mehanike loma od mikrostrukture jekla. Te parametre je moč privzeti kot efektivne, kar je na koncu članka prikazano s primerom izračuna primarnega dela plinovoda iz malolegiranega jekla.

1 Introduct ion

In steel s t ructures good weldable low-carbon and low-alloyed steel wi th yield stress 200 to 800 MPa are used. In the recent t ime the res is tance of those steels to brittle fracture is m o r e f requent ly cs t imated by parameters which caracterize the crack s tab i l i ty 1 , 2 . Steel resistance to brit-tle f racture is h ighly dependan t on its microstructure. Thus the mechanica l proper t ies of steel, especial ly the parameters determining the crack stability, are use fu l in est imating the resistance to brit t le f rac ture . Th i s is especial ly valid when the value of those parameters is micros t ructura l ly a highly sensitive value.

It is very advan tageous if those parameters are applica-ble in des igning structures. This condi t ion is fulfi l led the

more comple te ly the better deser ip t ion of f rac ture condi-t ions is achieved in this way.

Paper deser ibes the decis ive e f fec t ive parameters of crack stability which are highly dependen t on micros t ruc-tures as a measure of u se fu l proper t ies of structural steel, and they can be also s imply appl ied in engineer ing design of structures vvhich rnust be resistant to brittle f rac ture .

2 Testing of Steel

Plates of ali structural steel types being used in fo rmer So-viet Union were invest igated wi th a special emphas i s on those s tandardized in G O S T 27772-88 . Steels d i f fe r by their composi t ion , s trength, and way of manufac tu r ing .

Stee l s vvith y ie ld s t resses 2 3 0 to 285 M P a are cha rac te r -ized as steel vvith s t a n d a r d s t r eng th , wh i l e h ighe r - s t r eng th s tee l s h a d y ie ld s t resses 2 9 0 to 375 M P a , and h igh- s t r eng th o n e s a b o v e 3 9 0 M P a . A c c o r d i n g to c h e m i c a l c o m p o s i t i o n the t r ea ted s tee l s c a n b e d i v i d e d in fou r g r o u p s :

• lovv-carbon s tee ls vvith u p to 0.22% e a r b o n , • l o w - a l l o y e d s teels , m a i n l y m a n g a n e s e - s i l i c o n o n e s

( 1 2 G 2 S , 0 9 G 2 S , 14G2) , • m a n g a n e s e m i c r o a l l o y e d s teels ( 1 4 G 2 A F , 09G2FB, . . . ) , • c o m p l e x m o l y b d e n u m - a l l o y e d s teels ( 1 2 G N 2 I F A J U ) .

C l a s s i l i c a t i o n a n d c o m p o s i t i o n of those s teels is par t ic -u la r ly d e s e r i b e d in r e f . 2 . T h e g u a r a n t e e d va lue s of y ie ld s t r e s ses of the d i s c u s s e d s tee ls a f t e r s t andard vvorking or h e a t - t r e a t m e n t p r o c e s s e s are d e s e r i b e d in Table 1.

Table I. Guaranteed values of yield.stresses of investigated structural steels

Stee l

G u a r a n t e e d y ie ld s t ress ( M P a )

S tee l

H o t ro l led N o r m a l i z e d

H ar de n e d

and

t e m p e r e d

Lovv-ca rbon

M a n g a n e s e - s i l i c o n

M a n g a n e s e m i c r o a l l o y e d

C o m p l e x m o l y b d e n u m

a l loyed

2 3 0 - 2 8 5

3 3 5 - 3 7 5

2 3 0 - 2 8 5

3 3 5 - 3 7 5

3 9 0 ^ 1 4 0

285

390

4 9 0

5 9 0 - 7 4 0

1 VT = ' K j e

a r

a n d in the čase of p l ane - s t r a in s ta te equa l to:

1 Ki rT =

6ir U T

( 1 )

(2)

In the c o n d i t i o n s of ae tual s t ress and strain s tates dur -ing the tes t ing , the s ize of p las t ic d e f o r m a t i o n z o n e is be -tvveen the tvvo e x t r e m e s . T h e cond i t i ons in ou r test ing cor-r e s p o n d e d to the fol lovving s ize of p las t ic d e f o r m a t i o n zone :

(-) due to lovv resistance to brittle fracture, the steel is not used in sueh a state

S tee l s of the th i rd g r o u p (e.g. 0 9 G 2 F B ) are u sed e i ther as c o n t r o l l e d b y ro l l ed , o r l ike s o m e o the r s of that g r o u p as t h e r m o m e c h a n i c a l l y t r ea ted by v a r i o u s p roces se s de ta i l ed d e s e r i b e d e l sevvhere 2 .

3 I n v e s t i g a t i o n M e t h o d s

E s t i m a t i o n of the c r a c k s tabi l i ty in due t i l e s t ructura l steel h a s s o m e spec ia l t i es . I nc r ea se of n o m i n a l s t ress can e x c e e d the y ie ld s t ress o n the c r a c k f ron t in these steels. In ali c a s e s it h a p p e n s b e f o r e the s t ress in tensi ty fac tor r e aches its c r i t ica l va lue .

At that t ime p las t ic d e f o r m a t i o n t akes p lače at the c rack tip in the z o n e of r j s ize (Fig . 4) . T h e s ize of z o n e is fo r the č a s e of p l a n e - s t r e s s e d s tate equa l to:

vt « 0.1 A7c (Tj

Figure t . CTS test probe (type 3 according to GOST 25.506-85) for determining crack stability parameters in structural steel.

I = (0 .45 to 0 . 5 5 ) S , ( = 0 . 5 B , C = 1 . 2 5 S , d = 0 . 2 5 B , K < 0 . 0 5 B , F = 0 . 5 5 B , H = 1 . 2 B

Slika 1. Epruveta CTS (tip 3 po G O S T 25.506-85) za merjenje parametrov stabilnosti razpoke v jeklih za gradbene konstrukcije.

I = (0 .45 do 0 . 5 5 ) B . t = 0 . 5 B , C = 1 . 2 5 B , d = 0 . 2 5 B . K < 0 . 0 5 B . F = 0 . 5 5 S , H = 1 . 2 B

C o r r e c t d e t e m i i n a t i o n of the f r a c t u r e t o u g h n e s s va lue (s t ress in tens i ty f ac to r ) is p o s s i b l e on ly if the size of r j p las t ic d e f o r m a t i o n z o n e is e s sen t i a l ly s m a l l e r t han the c rack length and e f f e c t i v e test p r o b e c ros s see t ion . In s t ruc-tural steel sueh a ra t io ( i ' t / 1 ) c a n n o t b e eas i ly a c h i e v e d s ince they h a v e lovv y i e l d s t r e s ses a n d h i g h f r a c t u r e t ough-nes se s K j e - B a s e d o n n u m e r o u s tes ts de t a i l ed d e s e r i b e d elsevvhere 2 s o m e g e n e r a l r e c o m m e n d a t i o n s fo r se lee t ion of t e s t -p robes fo r s tat ic t es t ing d e p e n d i n g o n p la te t h i ckness and steel s t reng th vvere g i v e n . F o r 4 0 to 6 0 m m pla tes of l ow-a l l oyed steel the K j e p a r a m e t e r c a n be co r rec t ly de-t e r m i n e d at t e m p e r a t u r e s belovv — 4 0 ° C vvith C T S p robes (Fig . 1). Fo r 20 to 4 0 m m pla tes the c o n t o u r e d doub le -can t i l eve r d o u b l e a x i a l l y n o t e h e d p r o b e (F ig . 2) g i ve s good resul ts s ince plas t ic d e f o r m a t i o n at c r a c k f r o n t is h ighly r e d u c e d in it. In m a n y cases , e s p e c i a l l y in indus t r ia l test-ing , the cyl indr ica l p r o b e s vvith c o n c e n t r i c pe r iphe ra l no teh (Fig. 3) g a v e g o o d resul ts .

M e a s u r e m e n t s vvere m a d e c o r r e s p o n d i n g l y to G O S T 2 5 5 0 6 - 8 5 s t anda rd ( D e t e r m i n a t i o n of cha rac te r i s t i c s of c rack s tabi l i ty in s tat ic l o a d i n g ) a n d to A S T M s tandards . R u p t u r e c o n d i t i o n s w e r e in t ens i f i ed b y d y n a m i c load ing and us ing c o r r o s i o n m e d i a . D y n a m i c tes ts vvere m a d e accord-ing to A S T M s t a n d a r d s 5 and to R D 5 0 - 3 4 4 - 8 2 ins t ruc t ions fo r m e t h o d se lee t ion ( D e t e r m i n a t i o n of cha rac te r i s t i c s of f r ac tu re t o u g h n e s s in d y n a m i c load ing) . Test vvere made bo th vvith C T S and C h a r p y p r o b e s vvith f a t i g u e c r a c k ac-co rd ing to G O S T 9 4 5 4 - 7 8 . I n f l u e n c e of c o r r o s i v e m e d i u m o n the c rack s tabi l i ty w a s s tud ied vvith p r o b e s a c c o r d i n g to

> < 45"

.20

Figure 2. Contoured double-canti lever double axially notched probe. Slika 2. Dvojno konzolno vpela klinasta epruveta z dvema osnima

zarezama.

UD

0Dk

Figure 3. Cylindrical probe vvith concentric fatigue crack—(type 2 according to G O S T 25.506-85). L = length betvveen the clamped

parts of probe in llie tensile testing machine. L = 5 D ; d = (0.6 to 0.7)D: Li > 7D; /0 = 0.5(D - d) > h + 1.5 mm;

/o > 3.71 g a; DK = D - 2h = (0.65 io 0.85)D. Sl ika 3. Ci l indrična epruveta s koncentr ično utrujenostno

razpoko—(lip 2 po G O S T 25.506-85). L = razdalja med deloma epruvete, ki se vpneta v trgalni stroj L = 5D\ d = (0.6 do 0.7)D;

L\ > 7D: /0 = 0.5(D - d) > h + 1.5 mm; (0 > 3.7 tg a\ DK = D - 2h - (0.65 do 0.85)D.

200

L | a CL

- 1 2 0 " 8 0

T ( ° C )

- 4 0

Figure 5. Fracture toughness of structural steel plate. Dependance of 1\ i c on temperature, plate thickness 2 0 m m . Probe f rom Fig. 2. 1) Hardened and tempered molybdenum-al loyed steel: 0 .12% C, 0 .54% Si. 1.05% Mn, 0.5% Cr, 1.47% Ni, 0 .12% V, 0 .24% Mo, 0 .011% Al,

0 .022% N, 0 .025% S 2) The same steel, plate thickness 40 m m , Rp = 7 1 0 MPa 3) Hardened and tempered manganese-s i l icon steel

(0.1% C, 1.48% Mn, 0 .9% Si, 0 .031% S, 0 .021% P); Rp = 4 3 5 MPa 4) Steel above, rolled; Rp = 3 5 0 MPa 5) Hot rolled low-carbon steel

(0.16% C, 0 .24% Si, 0 . 6 5 % Mn, 0 .025% S. 0 .025% P); Rp = 2 6 5 MPa.

Slika 5. Lomna žilavost pločevine iz jekla za gradbene konstrukcije. Odvisnost / \ / c od temperature, debelina pločevine 20 mm. Fpruveta slika 2. 1) Pobol jšano legirano jek lo z mol ibdenom: 0 .12% C, 0 .54% Si, 1.05% Mn, 0 .5% Cr, 1.47% Ni, 0 .12% V, 0 .24% Mo, 0 .011% Al,

0 .022% N, 0 .025% S 2) Isto jeklo, debelina pločevine 40 m m , Rp = 7 1 0 MPa 3) Pobol jšano mangan-si l ic i jevo jek lo (0.1% C,

1.48% Mn. 0.9% Si, 0 .031% S, 0 .021% P); Rp = 4 3 5 MPa 4) Jeklo (3) valjano; Rp = 3 5 0 MPa 5) Vročeval jano maloogl j ično jeklo

(0.16% C, 0 .24% Si. 0 .65% Mn, 0 .025% S, 0 .025% P);

265 MPa.

Figure 4. Scheme of crack tip vvith the zone of plastic deformation.

Sl ika 4. Shema razpoke s cono plastične defomiaci je .

G O S T 9 4 5 4 - 7 7 in d i s t i l l e d vva te r a n d i n 3 % N a C l vva te r s o l u t i o n a c c o r d i n g t o R M S E V n i e t h o d ( C o r r o s i o n p r o t e c -t i o n in b u i l d i n g e n g i n e e r i n g . C o r r o s i o n c r a c k i n g o f h i g h -s t r e n g t h a r m a t u r e s t e e l . I n v e s t i g a t i o n m e t h o d s , 1 9 8 6 ) .

M o v i n g r a t e o f t h e t e n s i l e - t e s t e r c l a m p i n g j a v v s vvas 2 • 1 0 " 8 m m / m i n vvh ich vvas s u f f i c i e n t f o r c o m p l e t i n g t e s t in o n e d a y . A l s o i m p a c t t o u g h n e s s vvith U a n d V - n o t c h e d p r o b e s ( a c c o r d i n g t o G O S T 9 4 5 4 - 7 8 ) vvas m e a s u r e d s i m u l -t a n e o u s l y vvith t h e u n i a x i a l l y l o a d e d t e n s i l e t e s t s a c c o r d i n g t o G O S T 1 7 9 7 - 8 4 vvith ( la t p r o b e s o f t h e s a m e t h i c k n e s s a s i n v e s t i g a t e d p l a t e .

4 R e s u l t s o f T e s t s

S t a t i c t e s t i n g g a v e a s e r i e s o f r e l a t i o n s v a l i d f o r t h e c r a c k s t a b i l i t y in s t r u c t u r a l s t e e l ( F i g s . 5 t o 8 ) . It s h o v v s t h a t i n s t e e l o f s t a n d a r d p u r i t y a n d r a t i o n a l m i c r o s t r u c t u r e t h e f r a c -t u r e t o u g h n e s s v a l u e i n c r e a s e s vvith t h e i n c r e a s e d s t r e n g t h a n d vvith t h e t r a n s i t i o n f r o m f e r r i t e - p e a r l i t e m i c r o s t r u c t u r e

POVRŠINA

CONA PLASTIČNE DEFORMACIJE

C O N A PLASTIČNE DEFORMACIJE

VRH R A Z P O K E

VRH R A Z P O K E

If

- 1 9 6 - 1 2 2 - 8 0

T ( °C)

Figure 6. Fracture toughness and the size of plastic deformation zone in normalized steel (0.17% C, 1.56% Mn, 0.4%, Si, 0.11% V.

0.015%. N, 0.008%- S. 0 .07% P). Sulphide inclusions are modified by addition of RF., curve (1) vacuum treated steel; R,, = 460 MPa. Slika 6. Lomna žilavost in velikost cone plastične defomiaci je v normal iz i ranem jeklu (0.17% C, 1.56% Mn, 0.4% Si, 0.11% V,

0.015%. N, 0 .008% S. 0 .07% P). Sullidni vključki so modificirani z dodatkom RZ, (1) jek lo vakuumirano; R p = 4 6 0 MPa.

5 600 650 680

T ( ° C ) Figure 7. Dependance of fracture toughness and the size of plastic

deformation zone on tempering temperature for molybdenum-al loyed steel (0.10% C, 0 .37% Si, 1.16% Mn, 3.1% Cr, 1.0% Ni. 0 .34% Mo.

0 .016% S. and 0 .04% P), plate thickness 20 m m . probe type f rom Fig. 3.

Slika 7. Odvisnost lomne žilavosti in velikosti cone plastične deformacije od temperature popuščanja za j ek lo legirano z

molibdenom (0.10% C. 0 .37% Si, 1.16% Mn, 3.1% Cr. 1.0% Ni. 0.34% Mo, 0 .016% S in 0 .04% P) debelina pločevine 20 mm.

epruveta si. 3.

to micros t ruc tures obta ined by hardening and temper ing (Fig. 5).

Plate th ickness reduces the A ' / e value (Fig. 5) due to the r educed plastic de fo rma t ion zone (? ' r ) . The investiga-tion results also indicate that f racture toughness of high-s t rength steel depends on type, amount and distribution of non-meta l l ic inclusions, main ly sulphides (Fig. 6). Pure steel (0 .008% S) excel ls the steel vvith standard amount of su lphur both in the respect of f racture toughness and in size of plast ic de fo rma t ion zone at crack tip ()••/•)• This conf i rms the inf luence of inclusions, on vvhich decohes ion takes plače ( fo rmat ion of voids) , on the condi t ions of crack initiation *.

In heat- t reated steels the fracture toughness A ' / c is abrupt ly reduced if t emper ing temperature is reduced f r o m 650 to 6 0 0 ° C (Fig. 6). The reason is in changed mech-an ism vvhich cont ro ls the crack initiation. The tvvo-stage process connec ted to fo rmat ion of microvoids is substi tuted by an energy u n d e m a n d i n g m e c h a n i s m of local destruetion vvhich is deta i led deser ibed elsevvhere4 .

T h e reduced temper ing tempera ture reduces the I\ j <• value measu red by static loading (Figs. 6 and 7) vvhich is in cont rad ic t ion vvith the hi therto ideas, especially vvith the changed size of plast ic de fo rma t ion zone r j .

Th i s can be expla ined by applied testing methods vvhich did not allovv a suitably h igh microstructural sensitivity of pa ramete r s deser ib ing the crack stability in ducti le steel vvith rat ional micros t ruc ture . This is conf i rmed vvith tvvo ad-di t ional cases of unsuf i ic ient microsh-uctural sensitivity in es t imat ing A ' / c va lue vvith static tests. Table 2 presents the relat ion betvveen the f racture toughness of manganese-si l icon steel and the chemica l contposi t ion and the harden-ing tempera ture . Steel santples vvith various chemical com-posi t ions vvere ha rdened at opt imal temperature of 9 3 0 ° C , and at 1050°C. Th i s tempera ture vvas chosen in order to de te rmine the inf luence of overheat ing. In ali the cases the steel samples vvere t empered at 650° C.

Invest igat ion results indicated that concent ra t ions of al-loying e lements had a small inf luence o n mechan ica l prop-erties. Overheat ing of steel h ighly deter iora tes the Charpy-test values, transition the impact toughness value vvhich vvas practically halved. K k; va lue is not extra h ighly influenced by overheat ing; the obta ined d i f fe renc ies vvere belovv 5% and they are in the region of measur ing errors.

The second čase is connec ted to the seleet ion of thermo-mcchanica l t reatment in manufac tu r ing 50 m m plate vvith yield stress Rp > 4 5 0 M P a m a d e of microa l loyed man-ganese steel (Table 3). Steel vvas quenched f r o m the rolling temperature and tempered at 6 5 0 ° C . Initial and final rolling temperatures , and the quench ing tempera ture (i.e. interval betvveen ftnished rolling and quench ing in vvater) vvere var-ied. Specif icat ions T M T 1, 2, 3, and 4 in the ment ioned table represent var ious regintes of t he rmomechan ica l treat-ment . 1\ j c va lues vvere measu red vvith C T S probes (Fig. 1) having plate thickness. T h e highest t empera tu re at vvhich the A ' / c value vvas correct ly measu red vvas —40° C. Table gives the dissipat ion of results of three tests. Table also suggests the seleetion of opt imal r eg ime of t reatment vvhich enables the yield stress above 4 9 0 M P a at s imul taneously the highest toughness transit ion tempera ture , i.e. TMT-1.

Simul taneously it is evident that f racture toughness val-ues ( A ' / c , - 4 0 and A " / c , - 7 o ) do not enable to j u d g e vvhich thermomechanica l t reatment is opt imal . These measure-ments only reliably indicate that T M T - 4 t rea tment (hot roll ing) is the mos t unus i tab le one. The men t ioned cases shovv that parameters of l inear f rac ture m e c h a n i c s are mi-c r o s t r u c t u r a l ^ not enough sensi t ive to enable the regarding seleetion of tested steels.

The most probable reason is the high ductil i ty of struc-tural steel; in this čase the ducti l i ty of high-s t rength steel vvhich have overs ized plasticity zone at crack tip in static

P.D. O d e s s k i j . N . K u d a j b e r g e n o v , L. K o s e c . F. Krž ič : R e s i s t a n c e of S t ruc tu ra l S tee l to C r a c k F o r m a t i o n a n d P r o p a g a t i o n

Table 2. Some parameters of bnttle-fracture resistance of manganese-silicon steels with yield stress Rp > 390 MPa, plate thickness 20 mm

Chemica l compos i t ion of steel

% Quench ing

temperature

( °C)

Mechan ica l proper t ies Chemica l compos i t ion of steel

% Quench ing

temperature

( °C) Rv

(MPa)

KCV~ , u

( J / c m 2 ) ( °C)

T ' — ' UXJ" JXIC ( M P a m 1 / 2 ) C M n Si S P

Quench ing

temperature

( °C) Rv

(MPa)

KCV~ , u

( J / c m 2 ) ( °C)

T ' — ' UXJ" JXIC ( M P a m 1 / 2 )

0 .09 1.42 0 .28 0 .030 0 .022 930 429 64 -20 165 0 .09 1.42 0 .28 0 .030 0 .022 1050 432 29 +20 160 0.09 1.30 0 .60 0 .022 0 .019 930 417 84 180 0.09 1.30 0 .60 0 .022 0 .019 1050 414 41 +20 175 0 .09 1.42 1.03 0.030 0 .023 930 435 69 -20 175 0 .09 1.42 1.03 0.030 0 .023 1050 445 41 +20 170

j—Toughness transition temperature determination was based on 50% tough fracture n — P r o b e in Fig. 2. —70°C was the higliest temperature on which A ' / c could be estimated

Table 3. Mechanical properties of 50 mm thick plate of microalloyed manganese steel (0.19% C. 1.58% Mn. 0.48% Si, 0.07% V, 0.024%. S)

Steel t reatment Rv (MPa) XXX

(°C)

7^50

<°C)

K C V"70

( J / cm 2 )

A - 4 U ic A ' " ™ I c Steel t reatment Rv (MPa)

XXX

(°C)

7^50

<°C)

K C V"70

( J / cm 2 ) ( M P a m1 / ' - ' ) Hot rolled 420 -70 -20 4 9 5 5 - 7 6 4 2 - 5 5 Hardened and tempered 500 -60 +20 31 6 1 - 8 2 5 2 - 6 7 T M T 1 520 -100 -40 92 6 7 - 1 0 3 5 2 - 6 4 T M T 2 570 -60 -20 38 5 2 - 9 1 4 2 - 6 4 T M T 3 620 0 32 6 4 - 1 0 6 3 9 - 6 7 T M T 4 635 -20 +20 28 4 5 - 6 4 2 7 ^ 8

i n — D e t e r m i n e d by impact toughness criterion .39 J

loaded p robes though ali the testing condi t ions descr ibed by corresponding s tandards were fulf i l led.

Micros t ruc tura l depcndance of investigated parameters becomes m o r e p ronounced in m o r e severe testing condi-tions vvhich r educe the extent of plastic de format ion and the size of plast ic de fo rma t ion zone at the crack tip. This occurs dur ing test ing in corros ive media (Fig. 7). In these tests the K \ c value increases vvith the increased temper ing temperature of steel. The obta ined result is the consecjuence of a h igh dens i ty of d isordered dis locat ion loops. Such a structure is essent ia l ly less resistant to stress cor ros ion 2 , especially if hyd rogen embri t t lement is developed. In steel vvith such a s t ructure , the KCJC value is essential ly lovver than the A ' / c o n e (Fig. 8).

Behaviour of ha rdened and tempered steel dur ing load-ing is charac te r ized by its substructure vvhich is fo rmed in recovery of ferr i te and in the beginning of recrystall ization. Materials vvith such a structure are not sensitive to stress corrosion; the re fo re there it is valid: A ' / c — ^ / c (Fig. 8).

Another w a y to increase the structural sensitivity of A'/C 3 1 6 the impact tests vvhere loading rates are increased for six orders of magn i tude . In this čase the extent of plas-tic deformat ion is r educed to sui table amount , also size of plastic de fo rma t ion zone at the crack tip is abruptly reduced, and it b e c o m e s dependan t on steel microstructure. This finding can be conf i rmed vvith the invest igat ions of steel of big pipelines vvhich measu red A " / c values vvere close to the K i c values de te rmined by static test ing (Table 4).

In static loading the C T S probe vvas appl ied vvhile for impact tests Charpy test probe vvith fa t igue crack vvas used.

Steels vvith equal A " / c va lues exhibit the same sequence

£ <2

Figure 8. Dependance of fracture toughness and the size of plastic defonmation zone on tempering temperature for microalloyed

manganese steel (0.14% C, 1.64% Mn, 0.52% Si, 0.07% V, 0.007% N, 0.031% S, 0.013% P). K c

I C — f r a c t u r e toughness for tests in corrosive medium.

Slika 8. Odvisnost lomne žilavosti in velikosti cone plastične deformacije od temperature popuščanja za mikrolegirano manganovo jeklo. (0.14% C, 1.64% Mn, 0.52% Si, 0.07% V, 0.007% N, 0.031%

S. 0.013% P). K ' f j—lomna žilavost pri preizkusih v korozijskem mediju.

Table 4. Fracture toughness and the size of plastic deformation zone on crack tip

Steel t rea tment Plate thickness

Chemica l composi t ion

Grain size

RP /Oc rT K f c rd T

( m m ) C S (pm) M P a M P a 1 / 2 m m M P a 1 / 2 m m

Microa l loyed M n steel, ha rdened vvith A1N, 12 0 .17 0 .012 9 4 1 8 80 4 .2 13.7 0 .15 normal ized Microa l loyed M n steel, ha rdened vvith VN, 12 0 .13 0.011 6 425 80 4.1 22.6 0 .14 normal ized

M n steel a l loyed vvith small • amoun t s of M o and N b , 16 0 .13 0.011 4 590 84 3.1 37 0 .29 contro l led rol led

af ter impact testing. Invest igat ions also showed the advan-tage of cont ro l led rol led plates. Microstructural sensit ivity of f rac ture pa ramete r s vvas also essentially increased, espe-cially the K j c and va lues vvhich are in a good correla-t ion vvith the grain size.

5 Analys i s of Resul t s

T h e descr ibed testing results shovv that the A ' / c va lue m e a -sured at static loading is not sufficiently microstructural ly sensi t ive proper ty (Figs. 6 and 7, Tables 2 and 3). The needed sensi t ivi ty can be achieved by increased test sever-ity or vvith m o r e d e m a n d i n g tests applying corrosive media or impac t loads (Fig. 7, Table 4). Microstructural sensitiv-ity vvas increased if the extent of plastic de format ion vvas reduced . As a rule , at least three of the follovving con-di t ions mus t fulf t l led in that čase: temperature belovv 0 ° C , d y n a m i c tensile load, stress raisers must be present in struc-ture, d imens iona l fac tor (great cross section, and the like), and unsui tab le steel micros t ructure . In these cases the pa-ramete r s of crack stability measured in the condi t ions of h ighly l imited plastic de fo rma t ion give good descript ion of f rac ture condi t ions .

T h e s e pa ramete r s vvere vvell appl ied in engineering de-sign of s tructures. As an example , the crack propagat ion in the vvall of m a i n pipel ine will be described. Crack vvas initiated in the vveld on the line of fus ion penetrat ion inside the pipel ine , and then it propagated tovvards the extemal surface . T h e I \ ' / c va lues and the critical crack lengths l r

in the heat a f fec ted zones for var ious steel are revievved in Table 5.

Crit ical size of de fec t vvas calculated by express ion 3 :

vvhere p is pressure , R pipe radius, and t vvall thickness. T h e ob ta ined resul ts shovv that crack in manganese-

si l icon steel b e c o m e s uns table at low temperatures , and it starts spon taneous ly to propagate in the axial direct ion at relat ively smal l penet ra t ion into the pipe vvall. At those tem-pera tures the use of p ipes m a d e of the ment ioned steel is not allovved. Af t e r cont ro l led rolling the critical size of crack b e c o m e s greater than the vvall thickness. Thus the stable crack vvhich reaches the pipe sur face hinders the sponta-neous propaga t ion of crack. At the given temperature the p ipe m a d e of the third steel (cited in Table 5) is safe.

Table 5. Fracture characteristics of steel in heat affected zone of pipeline vveld

Steel t reatment Plate th ickness

,.<*<-(30) IC

,(-60) ' C

( m m ) ( M P a m 1 / 2 ) (mm)

Mn-Si steel, normal ized 12 24 3

Microal loyed (V) M n steel, control led rolled 17 81 24

M n steel, microa l loyed vvith V and Nb, control led rolled 14 97 32

Fatigue crack vvas normal to the plate surface

Determinat ion the f rac ture toughness at static load at -6 0 ° C ( A / c > 100 M P a n r / 2 ) for manganese - s i l i con steel indicates the safety of that steel, but unfor tuna te ly the prac-tical exper iences d id not conf i rm it. Thus the parameters of linear f racture mechan ic s measu red in the condi t ions of very limited plastic de fo rma t ion d id not exhibi t only high structural sensit ivity but they are also use fu l in engineering design of br i t t le-fracture res is tance in the cases vvhen they enough accurately descr ibe the m e c h a n i s m and condit ions for brittle f racture of certain steel.

6 References 1 Werkstoffkunde Eisen und Stahl; Teil 1; Grundlagen

der Festigkeit. der Zahigkeit und der Bruchs; Verlag Stahleisen, Diisseldorf, 1983

2 Tylkin M.A.. Bolšakov V.I., Odesskij P.D.: Struktura i svojstva stroiteFnoj stali; Moskva, Metallurgija. 19X3

3 Knott J.: Mikromehanizmy razrušenija i treščinostojkost' konstrukcionnyh splavov; Mehanika razrušenija. (prevod iz angl.), Moskva, Mir. 1979. (101-130)

4 Smith E., et al.: Lokalizacija plastičeskogo tečenija 1 treščinostojkost vysokopročnyh materialov. Mehanika razrušenija, (prevod iz angl.); Moskva. Mir. 1980. (124— 147)

5 Duffy A.R., et al.: Praktičeskie primery rasčeta na sopro-tivlenie hrupkomu razrušeniju truboprovodov pod davle-niem, Razrušenie, 5; Moskva, Masinostroenie. 1977. (146-209)

Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti

M. Gojič, M. Balenovič, Željezara Sisak "IRI" d.o.o., Sisak L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana L. Vehovar, Inštitut za kovinske materiale in tehnologije, Ljubljana L.J. Malina, Metalurški fakultet Sisak, Sisak

Recently great attention is given to investigations related to oil - and natural gas exploitation, especially from the aspect of material selection used for production of tubular equipment. In these conditions tubes destructions caused by mechanical damages are less frequent than destructions caused by various corrosion crack forms and especially corrosion cracks in sulphide conditions, with H 2 S as a dominant component. Results of mechanical properties and evaluation of tendency of seamless tubes in as rolled and heat treated condition, produced from medium-carbon Mn-V steel to hydrogen embrittlement by method of cathode polarization at the constant current density of 4.0 m A/cm2 are presented in this paper besides the above stated possible tube destruction forms in the conditions of oil - and natural gas exploitation. The fractographic analysis of fractured samples surfaces after cathode polarization was also performed by scanning electron microscope. It was found that tubes in the as rolled condition (no heat treatment) shovv great tendency to hydrogen embrittlement with the values of the calculated embrittlement index of 86.4%. By normalization (900" C/30 min. air) of tubes, resistance to hydrogen embrittlement vvas not improved compared with the as rolled condition. The heat treatment (quenching + tempering) resulted in a great resistance of tubes to hydrogen embrittlement with the calculated embrittlement index of 25.1%. The stated results are proved by the fractographic analysis of morphology of fractures that are explicitly brittle for tubes without heat treatment, but tough for quenched and tempered tubes. Therefore it can be concluded that from the aspect of mechanical properties and corrosion resistance the most suitable tubes for use in the oil - and gas wells are those quenched and tempered at high temperatures (70CP C).

Danas se u svijetu velika pažnja poklanja istraživanjima vezanim uz problematiku eksploatacije nafte i zemnog plina, posebno s aspekta izbora materijala koji se koriste za izradu cijevne opreme. Propadanja cijevi u tim uvjetima uslijed mehaničkih oštečenja su manje cesto nego propadanja usiijed raz Učiti h oblika korozijskih raspucavanja, a posebno korozijskog raspucavanja u sulfidnim uvjetima gdje je H 2 S dominantna komponenta. U radu su pored navedenih mogučih oblika oštečenja cijevi u uvjetima eksploatacije nafte i zemnog plina prikazani rezultati mehaničkih svojstava kao i ocjena sklonosti valjanih i toplinski obradenih bešavnih cijevi iz srednjeugljičnog Mn-V čelika prema vodikovoj krtosti metodom katodne polarizacije kod konstantne gustoče struje od 4.0 m A/cm2. Takoder je provedena na scanning elektronskom mikroskopu i fraktografska analiza prelomnih površina uzoraka nakon katodne polarizacije. Utvrdeno je da cijevi u valjanom stanju (bez toplinske obrade) pokazuju veliku sklonost prema vodikovoj krtosti s vrijednostima izračunatog indeksa krtosti od 86.4%. Normalizacijom (900' C/30 zrak) cijevi otpornost na vodikovu krtost nije se poboljšala u usporedbi s valjanim stanjem. Medutim toplinskom obradom (kaljenje + popušatnje) postignuta je velika otpornost cijevi prema vodikovoj krtosti s izračunatim indeksom krtosti od 25.1%. Navedene spoznaje dokazuje i fraktografska analiza morfologije preloma koji su za cijevi bez toplinske obrade izrazito krti cijepajuči dok su žilavi za zakaljene i popuštene cijevi. Prema tome, ovaj rad dokazuje da su s aspekta mehaničkih svojstava i korozijske otpornosti za primjenu u naftnim i plinskim bušotinama najpogodnije cijevi koje su zakaljene i popuštene na visokim temperaturama (70CPC).

1 Introduct ion

Due to ever increas ing d e m a n d for energy a great attention is given to inves t iga t ions related to p rob lems connected to oil and natural gas exploi ta t ion, especial ly to the selection of material for tubular equ ipmen t . Thir ty five years ago medium qual i ty steels having 400 M P a yield strength were used for tubes and pipes in natural gas and oil exploi tat ion from 1 0 0 0 - 3 0 0 0 meters depths . N o w a d a y s the exploi tat ion

proceeds not only in h igher dep ths ( 1 0 0 0 0 m ) but in m o r e severe condi t ions also because of the p resence of highly corrosive acidic gas (H2S, C O 2 ) , ch lor ides , h igh pressure and temperature , etc. The re fo re , mater ia l for tubular ware have to satisfy h igh requ i rements for bo th mechan ica l and corros ion propert ies.

Cracking of tubular equ ipmen t in the p resence of sul-phide is part icularly f requent cor ros ion p h e n o m e n o n . The

select ion of appropr ia te steel and the heat t reatment of fi-nal goods to obta in opt imal compromise of mechanica l and cor ros ion proper t ies are used as c o u n t e r m e a s u r e 1 - 6 .

D i f fe ren t poss ih le tube d a m a g e in oil and natural gas exploi ta t ion with special emphas i s on S S C C (sulphide stress cor ros ion c rack ing) as a f o rm of hydrogen embri t t lement is descr ibed .

S a m p l e s m a d e f r o m hot rolled seamless tube manufac -tured f r o m m e d i u m ca rbon low al loyed M n - V steel vvere in-v e s t i g a t e d Ca thod ic polar izat ion follovving heat t reatment carr ied out unde r laboratory condi t ions vvas used to deter-m i n e suscept ibi l i ty to hydrogen embr i t t lement and evaluate cor ros ion propert ies .

2 F o r m s of oil p ipe d a m a g e

Tubes and p ipes used in natural gas and oil exploi tat ion are k n o w n as Oil Coun t ry Tubular G o o d s (OCTG) . O C T G is d iv ided into tubing, cas ing and drill tube used in vertical d i reet ion for pumping , ex temal proteet ion and dril l ing, re-spectively. In a vvider sense O C T G includes also pipe line used for t ransport purposes main ly in horizontal direetion.

Desp i t e c o m p l e x stresses pure mechanica l failure is less f r equen t as c o m p a r e d to corros ion failure. Economic op-erat ion of oil vvells is o f t en dependent on proper selection of tube mater ia l . The control of corros ion has b e c o m e one of m a i n fac to rs in the p rodue t ion of energy f rom geoen-erget ic sources . Cor ros ion fai lure of natural gas and oil exploi ta t ion equ ipmen t is very ser ious p rob lem f rom safety as vvell as e c o n o m i c vievvpoint s ince the costs in oil indus-try a m o u n t to several hundreds mil l ions of USS per year. Mater ia l fai lure due to cor ros ion is also associated vvith p rodue t ion breaks. Cor ros ion p rob l ems in oil and gas ex-plota t ion e q u i p m e n t 7 ~ 1 0 can appear in several forms:

• vveight loss d u e to corros ion, • local ized corros ion knovvn as pitting, • co r ros ion fa t igue, • ga lvanic corros ion , • s t ress corros ion and • su lphide stress corros ion cracking (SSCC)

Stress cor ros ion cracking caused by sulphide is very f r equen t s ince typical oil and gas exploi tat ion envi ronment bes ides chlor ides , sulphates , carbonates , C 0 2 and mois ture con ta ins cons iderab le amoun t of H 2 S also vvhich can reach up to 30%. Hydrogen sulphide attack increases general cor ros ion , e ros ion cor ros ion in turbulent media , stress cor-rosion c racking , corros ion fat igue of tubes and equipment at bo t tom of dri l led vvells etc. Stress corrosion cracking in the presence of sulphide may appear in tvvo forms:

• hyd rogen induced cracking (HIC)

• su lphide stress corros ion cracking (SSCC)

H y d r o g e n induced cracking is characterist ic for O C T G equ ipmen t m a d e f r o m lovv al loyed steel vvith ferr i te—perl i te micros t ruc ture and 700 M P a tensile strength. It can occur even in the absence of external stress. This form of corro-sion resul ts f r o m atomic hydrogen absorbed on microstruc-tural defecLs (hydrogen traps) vvhich recombines into molec-ular hydrogen .

Su lph ide stress cor ros ion cracking occurs in O C T G equ ipmen t m a d e f r o m high s trength steel as a f o rm of hy-drogen embr i t t l ement . Hydrogen embri t t lement has been

vvell knovvn and f requent ly obse rved in metals for qui te a long t ime. It is caused mos t ly by corros ion , ga lvanisa t ion or leaching associated vvith the genera t ion of a tomic hydrogen vvhich under certain condi t ions can d i f fuse into crystal l ine lattice resulting in the hydrogen iza t ion of metal .

In the beginning the e f fec t of hydrogen vvas at tr ibuted to stress corrosion. Recent ly the similari ty betvveen hydrogen embri t t lement and certain types of stress corros ion, partic-ularly S S C C 8 - 1 0 has been pointed out.

There is no un ique theory capable of expla in ing ali phe-nomena associated vvith hyd rogen attack because it depends on a number of fac tors e.g. type of steel , its micros t ruc-ture, electrolyte, etc. At present , the p roposed mechan i sms of hydrogen attack are based on inerease in the inner pres-sure, surface adsorpt ion, decohes ion , inerease or decrease in plasticity and the fo rma t ion of h y d r i d e u . Acidic enviro-ment in natural gas and oil explo i ta t ion enhances hydrogen embri t t lement and S S C C as its par t icular f o r m because of

• the presence of H 2 S at lovv pH value of media ,

• sulphides vvhich inerease the amoun t of hydrogen dif-fus ing into the crystal l ine lattice of metal and

• tendency for the local izat ion of anodic part of corrosion reaction vvhich p romotes initial c racks .

There are tvvo sources of a tomic hydrogen ; the inner gener-ated by manufac tu re and heat t rea tment of steel and the ex-terior result ing f r o m the e f fec t of defini te env i ronment . Dur-ing applicat ion of mater ia l hydrogen m a y be adsorbed f rom gaseous phase in molecu la r f o rm vvith subsequen t dissoci-ation into a toms or by e lec t rochemica l d isso lv ing of liquid phase i.e. sur rounding corros ive med ia vvhich takes plače dur ing the exploi tat ion. In this čase hyd rogen is fo rmed in molecular or a tomic fo rm. The overal l react ion of hydrogen format ion is

in acidic solut ions: 2 H 3 0 + + 2 e ~ — H 2 + 2 H 2 0 (1)

in caustic solutions: 2 H 2 0 + 2 e ~ — H 2 + 2 0 H ~ (2)

The transport of H j O + ion ( f ront now on H + ) or H 2 0 moleculc to e lectrode sur face and subsequent fo rmat ion of adsorbed hydrogen a toms can be descr ibed by (3) and (4):

H + + e " - H a d s (3)

H 2 0 + e - - H a d s + O H - (4)

Irrespective on the nature of solut ion, hydrogen is ad-sorbed on metal e lectrode sur face . To be cont inuous the e lect rochemical process requires pe rmanen t presence of H a i s on e lectrode sur face f r o m vvhere it can be removed in one among three w a y s 1 2 , 1 3 :

• by catalyt ic recombina t ion (Volmer-TafeTs mecha-nism) vvhere both the adsorpt ion and desorp t ion take plače s imul taneously:

H + + e " - Haiis (5)

Hads + Hads — H 2 (6)

• by Volmer -Heyrowski ' s m e c h a n i s m of electrochemical desorpt ion vvhere desorp t ion results f r o m the reduetion

Table 1. Chemical composition of the Heat (T ) and billet (A )

Steel S a m p l e C o m p o s i t i o n in wt . % C M n P S Si V M o Al Cr

M n - V T 0 . 3 3 1.10 0 .017 0 . 0 0 4 0 . 2 3 0 .21 - 0 . 0 2 7 _ K 0 .33 1.14 0 .025 0 .005 0 .29 0 .23 0 . 0 2 0 .04 0 . 0 8

of H + ion or H 2 O m o l e c u l e accord ing to (7), (8) and (9):

H + + e"

H + + H a d s + e"

H 2 0 + H a d s + e -

Hads H 2

H 2 + O H "

( 7 )

(8)

(9)

• by e m i s s i o n m e c h a n i s m where adso rbed h y d r o g e n a t o m s v a p o r i z e f r o m e lec t rode su r face :

H Uds H ( 1 0 )

C o n s e q u e n t l y , h y d r o g e n a t o m s adso rbed on the su r face (H ads) c a n r e c o m b i n e in to e i ther h a r m l e s s g a s e o u s hy-d r o g e n t h r o u g h (6) vvhich is b u b b l e d out of solut ion or d i f f u s e into me ta l and e n h a n c e embr i t t l emen t :

Hads —* Hahs

It can be c o n c l u d e d that su lph ide stress cor ros ion c rack-ing is c a u s e d b y abso rbed h y d r o g e n . T h e d i s so lv ing of iron and the p r e s e n c e of H 2 S in vvater so lu t ion c rea te condi t ions for increase in h y d r o g e n con ten t of s t ee l 1 4 . Usual ly , on ly a small par t of h y d r o g e n gene ra t ed on ca thode d i f fu se s into metal . D i f f u s i o n rate d e p e n d s on n u m e r o u s faetors , e .g. the type of s teel or alloy, its c o m p o s i t i o n and p rev ious the rmo-mechan ica l t r ea tmen t , na tu re of e lec t rode su r face , type of e lect rolyte , its c o m p o s i t i o n , c a thode current densi ty, etc.

H y d r o g e n e m b r i t t l e m e n t is o f t en used in eva lua t ion of the e f f ec t of h y d r o g e n on steel at r o o m tempera tu re vvhich results in a loss of duc t i l i ty ( redue t ion in e longa t ion and contrac t ion) , d e c r e a s e in tens i le s t r eng th and enhanced brit-tleness.

3 E x p e r i m e n t a l

Mn-V steel bi l let of cfl 135 • 4 2 0 m m d i m e n s i o n s vvas pro-duced u n d e r l abora to ry cond i t ions o n Meta l lurg ica l Insti-tute H a s a n Brk i č , Zen ica . T h e billet vvas hot ro l led into 0 6 0 . 3 - 4 . 8 3 m m p u m p i n g oil tube ( tubing) under industr ial condi t ions in S e a m l e s s T u b e Mil i of Že l j eza ra (Iron and Steelvvorks) S i sak . Table 1 p resen ts Heat ( T ) and a control ( K ) ana lys i s of the steel .

S a m p l e s cu t f r o m rol led tube vvere sub jee ted to heat treatment ( annea l ing , annea l ing + temper ing , quench ing + temper ing) in e lec t r ic r e s i s t ance c h a m b e r fu rnace . Inves-tigation of m e c h a n i c a l p rope r t i e s vvere c a n i e d out on IN-S T R O N type 1196 m a c h i n e us ing s a m p l e s p repared accord-ing to A S T M s tandard . Br ine l l or Rockvvell C hardness vvas determined d e p e n d i n g o n the s a m p l e hardness .

Cor ros ion re s i s t ance vvas m e a s u r e d by the me thod of cathodic po la r i za t ion vvhich is knovvn as one a m o n g the most app ropr i a t e fo r d e t e r m i n a t i o n of re la t ive suscept ibi l i ty to hydrogen embr i t t l emen t . A f t e r c l eans ing vvith ace tone the samples fo r ca thod ic po la r i za t ion vvere put in the elec-trochemical cel i of ZWIC'K 50 k N tensi le m a c h i n e (fig. 1) and sub jee ted to stat ic load of 8 0 % of y ic ld stress.

1 - POSTOE .[ I M! MANI/ A n KIDAL 2- tajus 11 KioAuti -i • H I K I R (?K E 111 1 S K A r F L I JA - -UZORAK

0T0PINA 0 M n T LJ EI I KTR0DE

/ 1 IJGGINOVA KAPI| ARA REFLRENTNOM E LEKIR000M ISPIIŠIANJE OTOPINE P0KAZIVAČ ',111

10 -1' I '-.Ar 11 -REGULATOR BRZINI 12 ZAUSTAVI JANJE RA,7 VLAČ t N JA H IKLJUL IVAN JE RAZVLAEENJA ] i. -POVRAT Ni H0Q 1 S-U TE Z NA KI A I NU lb-PREKIU A POGONA , DA! IC E

ref elektioda proTuelektroda rodna elektroda

Figure 1. Schematic illustration of equipment for hydrogen emhriltlement evaluation by cathodic polarization method.

Slika 1. Shematski prikaz aparature za ocjenu vodikove krtosti metodom katodne polarizacije.

S a m p l e of M n - V steel vvas u s e d fo r vvorking e lec t rode and sa tura ted ca lomel e l ec t rode ( S C E ) s i tua ted in L u g g i n ' s capi l la ry tube as a r e fe rence . Tvvo g r aph i t e J o h n s o n M a t t h e y e lec t rode of 6.5 n u n d i a m e t e r vvere u sed as coun t e r e lec -trode.

T h e solut ion used vvas I N H 2 S 0 4 vvith add i t ion of 10 mg/ l A S 2 0 ? u s ed to ae t iva te the g e n e r a t i o n of H ad s- T h e so lu t ion vvas deae ra ted b y n i t r o g e n blovving fo r 3 0 m i n .

For ca thod ic po la r iza t ion W E N K I N G poten t ios ta t m o d -eli 68 FR 0 .5 vvas used and cons tan t 4 n i A / c m 2 cu r ren t den -sity vvas appl ied. A f t e r 2 hour s of po la r i za t ion s a m p l e s vvere taken out of the celi and i m m e d i a t e l y tes ted o n I N S T R O N m a c h i n e at very lovv d e f o r m a t i o n ra t e of 2 .4 - 1 0 ~ 4 s _ 1 . T h e overal l tensi le test las ted fo r 3 - 4 rnins. Aftervvard the c ross seet ion of f r ae tu red s u r f a c e vvas m e a s u r e d to d e t e r m i n e the cont rac t ion . D u e to the loss of duc t i l i ty ( con t rac t ion) e m -br i t t lement index F vvas ca l cu la t ed f r o m :

F = Z o

Zn — • 1 0 0

vvhere:

Z0

Z1 con t rac t ion b e f o r e po la r i za t ion

con t rac t ion a f te r po la r i za t ion

Table 2. Results of mechanical testing of Mn-V steel tube samples in as rolled and heat treated state.

Sample Heat treatment Re Rm A2" Hardness, HB: API MPa MPa % I II III IV grade

2 - 605.5 759.5 25.5 216 216 211 213 L-80

900°C/30 ' air 20 +

670° C/60 ' air 535.0 657.0 24.6 222 215 229 235 J-55

870°C/30 ' vvater 21 +

640° C/60 ' air 794.0 836.0 21.2 276 278 276 278 P-105

8 7 0 7 3 0 ' vvater 23 +

7 0 0 7 6 0 ' air 692.0 723.0 23.4 232 234 255 231 C-90

2 ?0/Jm I 1

— trakasta feritno-perlitna mikrostruktura — a) valjano stanje

— popušteni martenzit — b ) K : 8 7 0 " C / 3 0 v o d a + P : 7 0 0 ° C / 6 0 z r a k

Figure 2. Microstructure of tubing Mn-V steel in rolled (a) and heat treated (b) condition.

Sl ika 2. Mikrostruktura cijevi iz Mn-V čelika u valjanoni (a) i toplinsko obradenoni (b) stanju.

4 Results

Results of mechanical and metallographic investigation

Investigation of mechanical properties (tensile strength, yield stress and strain) vvere carried out on two tube sam-

ples in as rolled state and another tvvo in heat treated state. Ring-like 0 6 0 . 3 '1.83-30 m m samples vvere used for Brinell or Rockvvell C (for quenched samples) hardness measure-ment using three impressions per quadrant (in the middle of sample wall). Average values of mechanical properties for as rolled and different heat treated state are seen in table 2.

Mechanical properties of Mn-V steel tubes in as rolled state i.e. vvithout heat treatment correspond to L-80 API grade of corrosion resistant O C T G wares. Annealing treat-ment (900°C/30 ' in air) follovved by tempering at 670°C produces lovver quality O C T G vvares vvith mechanical prop-erties correponding to J-55 API grade. Quenching in vvater coupled vvith subsequcnt tempering at 640° C yields O C T G vvares of higher mechanical propert ies (P-105 grade) vvhich are not desired because of poor resistance to SSCC, i.e. a high susceptibility to hydrogen embri t t lement . Hovvever, tempering at 700°C results in API C-90 grade correspond-ing to corrosion resistant O C T G vvares.

Mechanical properties measured are in accordance vvith corresponding microstructures as can be seen on fig. 2.

Results of cathodic polarization tests

Since the determination of susceptibility to hydrogen em-brittlement by catodhic polarization is based on the loss of ductility caused by absorbed hydrogen, samples vvere sub-jected to tensile test vvith 2.4 • 1 0 ~ 4 s ~ ' deformat ion rate immediately after the polarization. Embri t t lement index F was calculated f rom equation (11) taking into account con-traction measured before (Zu) and af ter (Z\) polarization. The results are given in table 3.

Histograms given on figs. 3 and 4 present the change in contraction and embri t t lement index, respectively for as rolled and heat treated Mn-V steel sample caused by ca-thodic polarization. Samples in as rolled state and that in annealed state shovv high susceptibili ty to hydrogen embrit-tlement i.e. a great decrease in contract ion (fig. 3) and high index of embri t t lement (fig. 4). Samples annealed and subsequently tempered at 670° C have significantly lovver embrit t lement (F = 28 .1% only).

The best resistance to hydrogen embri t t lement ( F = 25 .1%) have samples vvhich vvere quenched and tempered at 700° C because of highly tempered martensi te microstruc-ture as seen in fig. 2b. Fracture surface of samples af-ter cathodic polarization vvas analysed by electron scanning microscope as seen in fig. 5. Fracture surface of Mn-V steel samples in as rolled state after cathodic polarization

100

8 0

6 0 -

40

2 0

0

[ Ž e

K + P VS N N + P

t o p L i n s k a o b r a d a

F i g u r e 3 . Concentralion change resulting from cathodic polarization of Mn-V steel in different heat treatment conditions VS-rolled. Heat

treatment

. N: 900° /30 ' air

. N + P 900° /30 ' air + 670° /60 ' air

. K + P: 870° /30 ' water + 700° /60 ' air S l i k a 3. Promjena kontrakcije uslijed katedne polarizacije za različita stanja toplinske ohrade Mn-V čelika. VS—valjano stanje. Toplinska

obrada

• N: 900° /30 ' zrak

. N + P 900° /30 ' zrak + 670° /60 ' zrak

• K + P: 870° /30 ' voda + 700°/60 ' zrak

100

8 0 -

6 0 -

4 0 -

2 0 -

VS N N + P K + P

t o p l i n s k a o b r a d a

F i g u r e 4. Embrittlement index change resulting f rom cathodic polarization of Mn-V steel in different heal treatment conditions

VS—rolled. Heat treatment

• N: 900° /30 ' air

• N + P 900° /30 ' air + 670° /60 ' air

. K + P: 870°/30 ' vvater + 700° /60 ' air S l i k a 4 . Promjena indeksa krtosti uslijed katodne polarizacije za

različita stanja toplinske obrade Mn-V čelika. VS—val jano stanje. Toplinska obrada

. N: 900°/30" zrak

• N + P 900° /30 ' zrak + 670° /60 ' zrak

• K + P: 870°/30 ' voda + 700° /60 ' zrak

shovvs typ ica l br i t t le f r a e t u r e ( f ig. 5a ) vvhereas q u e n c h e d and h igh ly ( 7 0 0 ° C ) t e m p e r e d s a m p l e s shovv due t i l e f r ae -ture (fig. 5 b ) c o i r e s p o n d i n g to a lovv index of embr i t t l emen t (fig. 4).

5 D i s c u s s i o n

Despi te the f ac t that m e c h a n i c a l p roper t i e s of M n - V steel seamless tube s a m p l e s in as ro l led s tate c o r r e s p o n d to A P I L-80 g r a d e of c o r r o s i o n res i s tan t O C T G vvares, the ca thod ic polar iza t ion at 4 . 0 m A / c n r cur ren t dens i ty d i sp l ayed grea t suscept ibi l i ty to h y d r o g e n embr i t t l emen t . A s a resul t of ca-thodic p o l a r i z a t i o n the c o n t r a c t i o n d r o p p e d f r o m initial (as rolled s ta te) 5 6 . 9 % to 7 . 7 % c o r r e s p o n d i n g to embr i t t l emen t index F = 8 6 . 4 % (f igs . 3 and 4) . T h e suscept ib i l i ty to hydrogen e m b r i t t l e m e n t of M n - V steel tubes in as ro l led state is a l so s een (f ig. 5a ) f r o m bri t t le f r ae tu re sur face . It resul ts f r o m s t r ip l ike fe r r i t e -per l i t e mic ros t ruc tu re vvith e longated i n c l u s i o n s (fig. 2a) vvhich is f avo rab l e for accu-mulat ion of the cr i t ica l a n t o u n t of h y d r o g e n requi red for the ini t ia t ion of c r a c k i n g . F r o m the vievvpoint of res i s tance to S S C C viz . h y d r o g e n e m b r i t t l e m e n t , e l o n g a t e d su lph ide inclusions ( e spec ia l ly M n S vvhich act a s h y d r o g e n trap) and high t e n d e n c y fo r s e g r e g a t i o n of n t a n g a n e s e (mar tens i t e and bainite i s l ands o b s e r v e d in m i c r o s t r u c t u r e ) a re u n f a v o r a b l e and have a d o n t i n a n t i n f l uence o n c o r r o s i o n res i s t ance of Mn-V steel in as ro l led s tate .

T h e annea l ing t r e a tmen t ca r r i ed ou t ( 900° C / 3 0 ' air) on tes ted tubes d id not i n t p r o v e the r e s i s t ance to h y d r o g e n e m b r i t t l e m e n t s ince s t r ip l ike f e r r i t e -pe r l i t e s t rue tu re vvith p reva i l ing in f luence of M n S inc lus ions ae t ing as h y d r o g e n t raps vvas p rese rved . O n the con t ra ry , t e m p e r i n g ( 6 7 0 ° C / 6 0 ' air) of annea l ed tubes resu l t ed in c o n s i d e r a b l e ine rease of the res i s tance to h y d r o g e n e m b r i t t l e m e n t s ince m a r t e n s i t e and ba in i te i s lands vvere r e m o v e d . In r e s p e c t to m e c h a n i c a l p rope r t i e s the hea t t r e a tmen t c o m b i n e d of q u e n c h i n g a n d t e m p e r i n g at t e m p e r a t u r e s vvithin 6 4 0 - 7 0 0 ° C r a n g e y i e l d e d J -55 , P - 1 0 5 and C - 9 0 ( tab le 2) A P I g r a d e s . C a t h o d i c p o -lar iza t ion test of tubes c o r r e s p o n d i n g to A P I C - 9 0 g r a d e shovved h igh r e s i s t ance to h y d r o g e n e m b r i t t l e m e n t vvith on ly 25.1%' r edue t ion in con t r ac t i on . T h e o b t a i n e d h igh res is-t ance to h y d r o g e n e m b r i t t l e m e n t is i l lus t ra ted also by fig. 5b shovving due t i le na tu re of f r a e t u r e su r f ace .

A s c o m p a r c d to s a m p l e s in a s ro l l ed s tate the m i -c ros t ruc tu re of hea t t rea ted s a m p l e t u b e s of M n - V steel ins tead of b a n d s of fe r r i te -per l i t e vvas c o m p o s e d of hon to -g e n e o u s h igh ly t e m p e r e d m a r t e n s i t e n t a r k e d by h igh duc -tility and c a p a c i t y fo r a c c u m u l a t i o n of h i g h e r a m o u n t s of ene rgy g e n e r a t e d e .g. b y Z a p p f e ' s m e c h a n i s m 1 1 of h y d r o -g e n embr i t t l emen t . S ince M n S vvas o b s e r v e d in hea t t rea ted s a m p l e s also, it is ev iden t that h igh ly t e m p e r e d m a r t e n s i t e r e d u c e s h a r m f u l i n f luence of M n S o n c o r r o s i o n p rope r t i e s of O C T G vvares.

Table 3. Loss of ducti l i ty of M n - V steel as de t emi ined by the m e t h o d of ca thod ic p o l a n z a t i o n .

S a m p l c H e a t t r e a t m e n t Re A p p l i e d ^ 0 Zi i F

M P a s t r e s s % % m A / c n r % 2 - 3 - 5 9 9 . 1 0 . 8 Re 5 6 . 9 7 . 7 4 . 0 8 6 . 4

2 0 N - 4 9 0 0 ° C / 3 0 ' a i r 5 9 4 . 5 0 . 8 Re 6 1 . 1 1 0 . 9 4 . 0 8 2 . 0

9 0 0 ° C / 3 0 ' a i r

2 0 - 5 +

6 7 0 ° C / 6 0 ' a i r

5 1 9 . 4 0 . 8 Re 6 7 . 2 4 8 . 3 4 . 0 2 8 . 1

8 7 0 ° C / 3 0 ' v v a t e r

2 3 - 4 +

7 0 0 ° C / 6 0 ' a i r

6 8 1 . 8 0 . 8 R , 7 0 . 1 5 2 . 5 4 . 0 2 5 . 1

— krti c i j epa juč i p re lom — Uzorak 2—3 (va l j ano s t an je )

— žilavi p r e lom — Uzorak 2 3 - 4 )K: 8 7 0 ° C / 3 0 voda + P: 700"C/60 zrak)

F igu re S. Frac ture m o i p h o l o g y of spec imens fo rm M n - V steel in rolled (a) and heat t realed (b) condi t ion a f t e r ca thod ic polar iza t ton .

Sl ika S. M o r f o l o g i j a p r e l o m a uzo raka iz M n - V čelika u va l j anom (a) i top l insko o b r a d e n o m (b) s tan ju n a k o n ka todne po la r izac i je .

S u m m a r v

Based on the invest igat ion of susceptibi l i ty to hydrogen em-bri t t lement of seamless M n - V steel tubes utilized in natural gas and oil industry the fo l lowing conclus ions can be de-rived.

• Mechan ica l proper t ies of M n - V steel tubes in as rol led state wi th h ig ly or iented ferri te-perli te micros t ructure co r respond to L-80 A P I grade.

• Bes ide J - 5 5 and P-105, C-90 API grade confo rming to co r ros ion resistant O C T G vvares was also attained by heat t rea tment (anneal ing + temper ing, quenchung + t emper ing) of tubes.

• Ca thod ic polar iza t ion of tubes in as rolled state shovved smal l res is tance to hydrogen embr i t t lement since em-br i t t lement index F vvas 86 .4%.

• The resis tance to hydrogen embr i t t l ement vvas not im-proved by anneal ing ( 9 0 0 ° C / 3 0 ' air) (F = 8 2 . 0 % ) as compared to as rolled state.

• Based on brittle f rac ture sur face revealed by fracto-graphic analysis of samples subjec ted to ca thodic po-larization it vvas es tabl ished that M n - V steel tubes in annealed or as rolled state are not sui table for the use in oil industry.

• High resis tance to hyd rogen embr i t t l ement p roven by compara t ive ly smal l embr i t t l ement index (F = 25.1%0 and ducti le na ture of f rac ture sur face vvas ac-quired by quench ing and temper ing at a high temper-ature (700° C).

• In respect to both mechan ica l and cor ros ion proper t ies M n - V steel tubes quenched and t emepered at a high temperature are sui table for the use in natural gas and oil vvells.

Chromizing of Iron

Difuzijsko kromanje železa

M. Jenko, A. Kveder, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana S. Spruk, L. Koller, IEVT, Teslova 30, 61111 Ljubljana

In the paper, the theoretical aspects of CVD processes of iron chromizing and the comparison vvith the PVD process, developed by the authors for professional electronic industry are presented.

V sestavku so podani teoretični vidiki CVD postopkov difuzijskega kromanja železa in primerjava s PVD postopkom, ki so ga ovtorji razvili za potrebe profesionalne elektronike.

1 Introduct ion

Corrosion is o n e of the mos t frecjuent and the most unde-sired p rocesses on the sur face of meta ls and alloys. Since corrosion is a sur face react ion, ali types of protect ive coat-ing mus t be involved to change the behaviour of metall ic componen t in the sur face composi t ion . This change can be ach ieved by addit ion of a d i f ferent material or in the form of outer skin, vvhich provides a barrier betvveen the body and the sur rounding corrosive med ium. The fo rm of coating is the mos t c o m m o n ; it includes paints, plastics, metals depos i t ed by electroplat ing etc. It is also possible to modi fy the chemica l compos i t ion of the surface to be pro-tected, by d i f fus ion of a suitable metal or an e lement into it vvhich vvill comb ine vvith the parent metal or alloy and pro-vide the requi red res is tance to the corrosive medium. Such formed su r face al loys are cal led d i f fus ion coat ings. The di-mensional change of the protected spec imen is smaller than the th ickness of the e f fec t ive sur face alloy and it may be neglected.

C h r o m i u m d i f f u s i o n — c h r o m i z i n g is probably one of the most versat i le types of d i f fu s ion coat ings and it is applied to achieve res is tance to corros ion , thermal oxidat ion and abrasion for i ron, steel, s tainless steel, nickel and its alloys, molybdenum, tungs ten and its alloy, etc.

2 Technological d e v e l o p m e n t of chromiz ing

The first a t t empts to ach ieve a ch romium rich surface on iron by the d i f f u s i o n process vvere m a d e by Kelly in 1923". Iron s p e c i m e n s vvere buried into ch romium povv-der and treated in reduc ing a tmosphere . A ch romium rich layer, about 125 / t m thick, fo rmed after 4 hours heating at 1300°C, vvas a pro tec t ing layer vvith good adherence to the under ly ing meta l , resistant to corrosion, as vvell as to thermal oxidat ion and , therefore , very interesting for vvide commercial use.

Similar invest igat ion vvas m a d e by H.S. Cooper in 19382. T h e process of ch romiz ing vvas applied in a re-ducting a tmosphere at the t empera tu re of 1300°C, lasting 24 hours; the th ickness of ch romized layer vvas 250 / im.

A h igh process ing t empera tu re vvas d isadvantage of both procedures.

The chromiz ing process has unde rgone considerable de-velopment changes over the years and it ha s been the subject of careful and deta i led studies. A m a j o r achivement vvas

the introduction of volati le hal ides. L.H. Marshal l devel-oped the first C V D (chemical vapour deposi t ion) p rocedure of chromiz ing , us ing the volat i le ha l ides at the process ing temperature of 1050° C.

Modern chromiz ing processes like D A L , B D S , etc. are based on the above ment ioned pr inc ip le 3 , 4 .

Simultaneously, the first exper imen t s of v a c u u m d i f fu -sion chromiz ing vvere p e r f o r m e d by Hicks as ear ly as 1932. Part icles of pure iron vvere bur ied in a c h r o m i u m povv-der and heated for 96 hours at 1200° C in a v a c u u m of 4 • 1 0 " 2 mbar. Eight years later, Corne l ius and Bol lenrath obtained similar resul ts in their exper iments ; c h r o m i u m con-centrat ion profiles vvere de t e rmined by the X-ray analy-sis. Further, this process vvas descr ibed by G o r b u n o v and Dub in in 1 2 .

In Slovenia the v a c u u m chromiz ing process ( P V D — physical vapour deposi t ion) has been deve loped at the In-stitute for Electronics and Vacuum techniques together vvith the Institute of Meta l s and Technologies 4 , 17 and has been used for d i f fus ion c h r o m i u m coat ing of iron parls of mag-netic circuit for minia ture relays.

3 Fe-Cr const i tut ion d iagram

The i ron-chromium const i tut ion d i ag ram is shovvn in Figure 1. At approximatel ly 1000°C, it can be seen that the austen-ite microstructure of the iron remains u n c h a n g e d until a con-centrat ion of approximate ly 12% c h r o m i u m is reached vvhen c h r o m i u m is deposi ted and it d i f fu ses invvards. At higher ch romium concentra t ions , the micros t ruc ture b e c o m e s fer-ritic; cont inuat ion of chromiz ing causes m o v i n g of the al-p h a / g a m m a boundary into interior. Dur ing cool ing the fer-rite surface layer remains unchanged , vvhile inner austeni te is t ransformed into ferrite. Th i s recrysta l l izat ion of inner region vvith less than 12% Cr causes that boundary vvith 12% Cr is good visible, F igure 2. T h e dep th to vvhich extends the 12%- Cr bounda ry is taken as the thickness of chromized layer, F igure 3.

Since the rate of d i f fus ion of c h r o m i u m is greater in fer-rite than in austenite, there is a rapid rise in the ch romium concentrat ion of the coat ing tovvards the surface , and be-yond the 12% Cr bounda ry there is a sharp concentra t ion drop at the ferr i te/austenite boundary. Gra in bounda ry di f -fus ion occurs too, but it has a little e f fec t on the coat thick-ness, Figure 4.

1900 1800 1700 1600

Figure 3. Microsections of vacuum chromized iron samples at: 1050°C (a) 3 hours, (b) 8 hours (c) 12 hours, 1100°C (d) 3 hours,

(e) 8 hours, (f) 12 hours, 1150°C (g) 3 hours . (h) 8 hours. Nital etched.

Slika 3. Metalografski posnetki vakuumsko kromanih vzorcev želeZa pri: 1050°C (a) 3 ure, (b) 8 ur (c) 12 ur, 1100°C (d) 3 ure. (e) 8 ur,

( 0 12 ur, 1150°C (g) 3 ure, (b) 8 ur. Jedkano z nitalom.

Effective coating

o o 1500

1400

1300

1200

1100 1000

900

800

700

600

3NS:

i* i i * i — — —

0 10 20 30 40 50 60 70 80 90 100

% Cr

Figure 1. I ron—Chromium diagram 5 .

Slika 1. Fazni diagram Fe—C 5 .

1 MRG: 750 IBIDIFF CR 1B,

Figure 2. a) Micro-section of vacuum chromized sample. A sharp o /phase boundary is visible (nital etched). b) Cr concentration

profile of the same sample. Slika 2. a) Metalografski posnetek vakuumsko kromanega vzorca

železa; vidna je ostra fazna meja a l fa /gama ( jedkano z nitalom). b) Koncentraci jski profil kroma posnet z elektronskim

mikroanal iza tor jem na istem vzorcu.

The alloyed layer is generally called a coating, but it must be clearly dist inguished f rom the eoatings produced by electroplating and spraying processes, since there is no dif fusion. The chromized coating represents an inseparable part of the treated specimen, the composi t ion is changing f rom the surface to the core.

4 M e c h a n i s m and kinetics of cromizing the iron

4.1 Chromizing technic/ue with volatilc halidcs

In these processes ch romium is brought to the surface of the iron heated to 900-1150° C as a gaseous compound, e.g. c h r o m i u m chloride, where it is deposited in atomic form by a chemical reaction.

O

12%

a y

Distance f rom surface Figure 4. Chromium concentrat ion profile in o - F e C r layer.

Slika 4. Gradient kroma v kromani plasti.

In many chromizing techniques, chromium chloride is applied and an a tmosphere containing hydrogen is main-tained in the reaction chamber .

The deposition of chromium on iron is described vvith the follovving equations:

Interchange

Reduction

Fe + C r C b = F e C l : + Cr

C r C l j + H 2 = 2HC1 + Cr

• Dissociation CrCl 2 = Cl 2 + Cr

(D

( 2 )

(•'I)

In the reaction (1) an a tom of iron is r emoved f rom the surface for each deposi ted ch romium atom. Since iron and chromium atoms are similar in vveight and size, there occur

only slight mass and dimensions changes of iron specimens after the treatment. The reaction is reversihle and the equi-librium ch romium concentrat ion at the surface depends on the relative vapour pressures of iron and chromium chlo-rides in gaseous phase.

React ions (2) and (3) are catalysed by the iron surface. Theoretically the surface chromium concentration may ap-proach 100 per cent, but since it is assumed that the cat-alytic activity of the iron surface drops with the increasing chromium content , the concentration of chromium is lim-ited. The mass and dimension change are equivalent to the amount of deposi ted chromium.

Generally, the volatile halides are used for transport of chromium atoms to the surface of iron, where they are ad-sorbed and d i f fuse imvards.

In F i g u r e 5 the layout of BDS (Becker, Daeves, Stein-berg) proeess, a typical C V D proeess, is shovvn.

2. Chromium migration from the surface inwards into the specimen expressed by the interdiffusion coefficient D:

D = 2 . 0 8 e x p ( - 2 4 3 0 0 0 / R T ) (7)

Termoregu la t ion

Oegassing

Sample

Figuro S. The layout of BDS (Becker. Daeves, Steinberg) proeess,

typieal C V D proeess, is shovvn.

Slika 5. Shematičen prikaz C V D - B D S postopka.

(b)

Vacuum chromizing

There are tvvo possible processes of supplying an iron sur-face vvith the ch romium atoms:

• t ransfer due to the close contact of iron surface and ch romium granulate enabling the surface diffusion of Cr

• absorpt ion of Cr vapour through the formed gaseous phase

In vacuum chromizing the grovvth of a — F e C r layer is controlled by tvvo processes:

1. The arrivai and condensation of C r atoms on the sur-face of the specimen given by the condensation rate w:

(4)

/ g c m - 2 s " 7 ( 5 )

vvhere ak is the condensat ion coefficient; pCr (mbar) is vapour pressure; M is Cr molecular mass and T is absolute temperature (K). The decisive quantity is pcr, and its temperature dependence is being described by

p = 1 1 . 7 4 3 e x p ( - 3 9 4 0 0 0 / / ž r ) / m b a r / (6)

(R is the gas constant in J K - 1 m o l - 1 )

0 2 10 12 14 t / h

Figure 6. Thickness of the chromized layer, d, and the mass inerease, W, as a funetion of chromizing time t.

a) experimental results b) calculated values. Slika 6. Debelina vakuumsko kromane plasti d in narastek teže W v

odvisnosti od časa t a) eksperimentalni rezultati b) izračunane vrednosti.

By increasing the temperature 7', p inereases more rapidly than D as the evaporat ion enthalpy of chromium A H e v a p = 394 k J m o r 1 is higher than the aetivation energy for the d i f fus ion E d i j = 243 k J m o l - 1 . This c i rcumstance leads to three dif-ferent a - F e C r layer grovvth rates.

(a) At lovv temperatures 950 < 6 < 1 0 5 0 ° C the slovvest proeess is the Cr condensat ion. Ali con-densed Cr a toms are transported immediately by dif fus ion f rom the surface invvards. Therefore the layer grovvth rate is linearly proport ional to the condensat ion rate w:

w = Dt or d = Vwt

(b) At high temperatures 0 > 1150°C , vvhen p is high enough, the slovvest proeess is the diffusion, leading to the parabolic law

w = Dt d = vVTDt

w (Trn

0 2 8 10 12 K t / h

Table 1. Values appl ied in the eva lua t ion of the th ickness . d, and u e i g h t inerease , W, of u - F e C r layers g iven in F igure 6b. D - i n t e r d i f f u s i o n eoef f i e ien t , p - equ i l i b r ium C r v a p o u r pressure , u>-condensation rate of c h r o m i u m , ta, da—critical t ime e o r e s p o n d i n g o - F e C r layer th ickness

w h e n the l inear growth rate changes into the parabol ic one .

° C 9 0 0 9 5 0 1000 1050 1100 1150 1200 1250

D (cm1,-1) 1 . 5 2 x 1 0 - 1 1 4 . 2 0 X 1 0 ~ u 1 . 0 7 x 1 0 " 1 0 2 . 5 6 x 1 0 ~ 1 0 5 .72 X 1 0 - 1 0 1 .21 x 1 0 " 9 2 . 4 3 x 1 0 ~ 9 4 . 6 5 x 1 0 - 9

p ( m b a r ) 1 . 0 X 1 0 ~ 7 8 . 0 X 1 0 " 7 3 . 8 x 1 0 - 6 1 .5 x 1 0 - 5 5 . 7 X 1 0 " 5 1 .9 X 1 0 " 4 5 . 9 x 1 0 " 4 1.7 x 1 0 - 3

W (gcm-2s-') 9 . 2 3 X t O " 1 0 7 . 2 4 X t O " 9 3 . 3 6 X 1 0 " 8 1.3.5 x 1 0 " 7 4 . 8 8 X 1 0 - 7 1 .62 X 1 0 " 6 4 . 8 S X 1 0 " 6 1 . 3 6 x 1 0 - 3

ta (S) 4 . 3 2 x 10 7 1 .94 X tO 6 2 . 3 x 1 0 5 3 . 4 x 10 4 5829 1150 247 59 .31

(h) 1200 538 .8 63 .9 9 .52 1.6 0 .319 0 .069 0 . 0 1 6 5

da (^m2) 362 .4 127.60 7 0 41 .9 25.8 16.6 10.9 7 .3

At these va lues of T the linear rate appear ing in the early s tages of growth cannot , be detected.

(c) In the in termedia te region 1050 < 0 < 1 1 5 0 ° C the th ickness of o - F e C r layer begins to inerease l inearly wi th t ime. The grovvth rate changes to a parabol ic low at the critical t ime t , which corre-sponds to the critical th ickness d.

T h e ca lcula ted grovvth rates of a - F e C r layers are shovvn in F igure 6b . Table 1 conta ins ali necessary data; p is ob ta ined f r o m the equat ion (6), w f r om the equat ion (5) a s suming a — 1 , D f r o m the equat ion (7), and da f rom the re la t ionships g iven in F igure 6a.

! GORBUNOV ? KUBASCHEVSKI 3 BRf WER

PAZUHIN 5 KOVALENKO 6 THIS W0RK 7 H 0 NIG

Preal = CiC2C3p (8)

vvhere:

c i

co takes in account the residual a tmosphere ; c 3 takes in account the sur face /granula te ratio.

5 Conc lus ion

The results of this invest igat ion shovv that none of C V D processes is suitable for the proteet ion of i ron parts for m a g -netic circuit in minia ture he rmet ic relays. For this purpose P V D process of v a c u u m chromiz ing vvas deve loped . With this procedure the m a x i m a l c h r o m i u m eontent of 15% Cr at the surface vvas obta ined, enough for corros ion proteet ion in corrosive media vvhich are d e m a n d e d by M I L - R - 3 9 0 1 6 and MIL-R-5757 . P V D process assures the op t imal magnet ic propert ies, a very lovv coercivi ty and a good weldabil i ty and additionally, it is an env i ronmen t f r iendly process .

1000 1/00 '100 v. .10 1800

F igu re 7. Tempera tu re d e p e n d e n c e of equi l ibr ium C r vapour p ressure p acco rd ing to var ious r e f e r e n c e s " - 1 6 .

Sl ika 7. P a m i tlak k r o m a v odvisnos t i od t empera tu re p o podatk ih razl ičnih a v t o r j e v 1 1 - 1 6 .

T h e real Cr vapour pressure p r eai is equal to the equi-l ibr ium pressure p on ly if the exper imenta l condit ions are ca re fu l ly chosen: PRA < 1 0 " 4 mbar, vvhile the surface of the c h r o m i u m granula te has to be as clean as possible and the rat io of spec imens sur face and the granulate amount mus t be adequate . If these condi t ions are not correctly cho-sen then prcai can be expressed by p mult ipl ied by three correc t ion coeff ic ients c i , C3 < 1:

A —tXI

H v-

1 | i |

-1

1 dvostopenjska rotacijska črpalka 2 r r .en ln ik s r e d n j e g a v a k u u m a

3 t u r b o m o l e k u l a r n a č r p a l k a

4 • v e n t i l

5 - m e r i l n i k v i s o k e g a v a k u u m a

b - i z o l a c i j a

7 g r e l e c

8 š k a t l a iz n e r j a v n e g a j e k l a

9 k r e m e n č e v a c e v

takes in account the port ion of oxidized sur-face of granulate;

Figure 8. A schemat i c d i ag ram of P V D — v a c u u m c h r o m i z i n g p r o c e d u r e 7 .

Sl ika 8. Shema t i čen pr ikaz P V D — v a k u u m s k e g a d i f u z i j s k e g a pos topka k r o m a n j a .

6 References 1 N.A. Lockington, in Corrosion, Vol. 2, Principles of

Applying Coatings by Diffusion, Buttervvorths, London 1978.

2 R.L. Samuel, N.A. Lockington, Met. Treat. 18. 354. 407, 440 (1951).

3 G. Becker, K. Daeves, F. Steinberg, Stahl und Eisen 61. 289 (1941).

4 M. Jenko, A. Kveder, R. Tavzes, E. Kansky, J. Vac. Sci. Technol. A3. 6. 2657(1985)

E. Kansky, M. Jenko. Vaeuum 37, 1/2, 81 (1987). M. Jenko. R. Tavzes. E. Kansky. J. Vac. Sci. Technol A5/IV 2685 (1987). A.H. Sully. E.A. Brandes, Chromium, Chpt. 7, 258, But-terworth. London (1967). H. Cornelius, s. R Bollenrath, Arch. Eisenhuttenwesen, 15. 145 (1941). L.C. Hicks, Trans. AIME. 113. 13, 163 (1934). T. P. Ho ar. E A . Croom. J. Iron Steel Inst. 169, 101 (1951). R.E. Honig, D.A. Kramer. RCA Rev. 30. 285 (1969). N.S. Gorbunov. Diffusion coatings on iron and steel. Is-rael program for Sci. translations. Jerusalem 1962.

13 V.A. Pazuhin, A.J. Fišer, Vakuum v metallurgii, Metal-lurgizdat, Moskva (1956).

14 V.E Kovalenko, Teplofizičeskie Procesy i Elektrovaku-umnye Pribory, Sovetskoe radio, Moskva (1957).

15 J.L. Margrawe, The Characterization of High Tempera-ture Vapours, John Wiley and Sons, New York (1967).

16 O. Kubaschewski, E.L. Evans, C.B. Alcock, Metallurgi-cal Thermochemistry, Pergamon, Oxford (1967)

1 ' M. Jenko, A. Kveder, R. Tavzes, Postopek za protikorozi-jsko ščitenje majhnih kosov iz čistega železa ob hkratnem doseganju najboljših mehkomagnetnih lastnosti. Številka patenta: P-19470.

KOVINE ZLITINE TEHNOLOGIJE, 26, 1992, 1 -4

1. Kronološko kazalo

Ažman Slavko: Razvoj in problematika mikrolegiranih jekel za petrokemično industrijo KZT 26 (1992) 1-2, 015-022

Tolar Miha, J. Lamut: Racionalizacija in optimiranje proizvodnje v jeklarni Bela . . . . KZT 26 (1992) 1-2, 023-029

Triplat Jože: Razvoj tehnologije izdelave nerjavnih jekel v Železarni Jesenice od leta 1984 naprej

KZT 26 (1992) 1-2, 030-033 Ploštajner Henrik, V. Prešern, G. Todorovič: Optimizacija tehnologije izdelave in odlivanja jekla v Železarni Store

KZT 26 (1992) 1-2, 034-037 Lamut Jakob, F. Pavlin, A. Poklukar: Taljenje vložka s plinom v jaškasti kupolni peči . KZT 26 (1992) 1-2, 038-040

Vižintin J.: Razvoj in pomen tribologije doma in v svetu . . . KZT 26 (1992) 1-2, 041-048

Legat Franc: Toplotna obdelava verig s poudarkom na indukciji KZT 26 (1992) 1-2, 049-052

Rodič Alenka, J. Žvokelj, F. Legat, S. Krivec: Vpliv kemi-jske sestave na lastnosti jekel za verige po toplotni obdelavi .

KZT 26 (1992) 1-2, 053-057

Vojvodič-Gvardjančič Jelena: Nizkotemperaturna meja uporabnosti mikrolegiranih jekel s stališča lomne mehanike. .

KZT 26 (1992) 1-2 , 058-062 Dobi D.: Uporaba instrumentiranega Charpyja pri razvoju jekel

KZT 26 (1992) 1-2 , 063-073

Kosec Ladislav, N. Igerc, B. Kosec, B. Godec, B. Urnaut: Temperaturna utrujenost j e k e l . . KZT 26 (1992) 1-2 , 074 -078

Kejžar Rajko: Oplemenitenje površin z navarjanjem in metalizacijo KZT. 26 (1992) 1-2 , 079 -084

Kolenko Tomaž, B. Glogovac, D. Novak, D. Žagar, B. Omejc: Konceptualna rešitev procesnega vodenja ogrevanja vložka v potisni peči KZT 26 (1992) 1-2 . 085-088

Leš P., F. Dover: Tehnologija valjanja radialno orebrenih cevi za prenos toplote KZT 26 (1992) 1-2 , 089-093

Krajcar Josip, V. Ferketič, D. Vukovič, A. Ivančan, J. Bu-torac, A. Iharoš: Površinske greške čeličanskog izvora na bešavnim ci jevima KZT 26 (1992) 1-2 , 094-096

Iharoš B.: Izrada normativno-tehnološke karte hladnog vučenja Celičnih cijevi KZT 26 (1992) 1-2, 097-099

Torkar Matjaž, B. Šuštaršič: Preiskava vodno atomiziranega prahu iz zlitine Nimonic 80A . . KZT 26 (1992) 1-2, 100-103

Šuštaršič Borivoj, Z. Lengar, S. Tašner, J. Holc, S. Be-seničar: Izdelava AlNiCo trajnih magnetov iz vodno atom-iziranih prahov KZT 26 (1992) 1-2 . 104-109

Rihar G. : Obdelava kovin z žarkovnimi izvori energije KZT 26 (1992) 1-2, 110-113

Božič Antonija: Problematika sežigalnih naprav KZT 26 (1992) 1-2 , 114-117

Renko M., A. Osojnik: Razvoj metod za analizo redkih zemelj v zlitinah s posebnimi lastnostmiKZT 26 (1992) 1-2, 118-122

Lovrečič Saražin Marko: Hamiltonski indeks grafa KZT 26 (1992) 1-2 , 123-124

Tehovnik F., B. Koroušič, V. Prešern: Optimizacija modi-fikacije nekovinskih vključkov v jeklih obdelanih s Ca

KZT 26 (1992) 1-2 , 125-130

Kurbos Mojca, J. Lamut, T. Kolenko, M. Debelak: Vpliv pogojev konti litja na lastnosti slabov

KZT 26 (1992) 1 - 2 , 131-133

Lesjak D., J. Lamut, V. Gontarev, A. Purkat, M. Debelak: Modelne raziskave v jeklarstvu. KZT 26 (1992) 1 -2 , 134-139

Kanalec Slavko, M. Tolar, J. Lamut: Racionalizacije porabe apna v jeklarni Bela KZT 26 (1992) 1 -2 . 140-142

Kert A., J. Apat, J. Lamut: Pretaljevanje sekundarnih surovin KZT 26 (1992) 1 - 2 , 143-146

Hajnže D., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini Fe in Si KZT 26 (1992) 1 -2 , 147-150

Drofenik B., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini železa in silicija mikrolegiram s selenom in kositrom KZT 26 (1992) 1 -2 , 151-155

Vehovar Leopold, M. Mavhar: Korozijska odpornost ner-javnega superferitnega jekla Acrom 1 super v primerjavi z avstenitnim nerjavnim jeklom Acroni 11 Ti

KZT 26 (1992) 1 - 2 , 156-164

Vehovar Leopold, M. Pečnik: Korozijska odpornost superzl-itine Ravloy 4 KZT 26 (1992) 1 - 2 . 165-168

Vehovar Leopold, T. Pavlin: Prepustnost mikrolegiranih jekel za vodik KZT 26 (1992) 1 -2 , 169-171

Smolej A.: Razvoj sodobnih zlitin aluminija KZT 26 (1992) 1 -2 , 172-177

Kostajnšek J.: Čiščenje talin z vpihovalnim rotorjem za uva-janje čistilnih plinov v talino. . . K Z T 26 (1992) 1 - 2 , 178-180

Kristan T.: Aluminij in avtomobilska industrija KZT 26 (1992) 1 -2 , 181-182

Doberšek M., I. Kosovinc: Vplivi indija na lastnosti dentalnih zlitin Ag-Pd-Aul-Cu-Zn5-In5 . . KZT 26 (1992) 1 -2 , 183-187

Segel Jože: Računalniška podpora krmiljenju proizvodnje . . . KZT 26 (1992) 1 -2 , 188-193

Koroušič Blaženko: Pomen matematičnega modeliranja pri študiju jeklarskih procesov KZT 26 (1992) 1 -2 , 194-196

Štok B., N. Mole: Numerična simulacija procesa izdelave in-gotov po EPŽ postopku KZT 26 (1992) 1 -2 , 197-200

Jenko Monika, F. Vodopivec, B. Praček: Raziskave segre-gacij na površini Fe-Si-C-Sb zlitin z metodo AES

KZT 26 (1992) 1 - 2 , 201-204

Smajič Nijaz: Dinamični model vakuumske redukcije VOD žlinder KZT 26 (1992) 1 -2 , 205-208

Šuštaršič Borivoj, M. Torkar, M. Jenko, B. Breskvar, F.Vodopivec: Procesi atomizacije kovinskih gradiv in konsol-idacije kovinskih prahov — II. del

KZT 26 (1992) 1 - 2 , 209-214 Rodič Jože, K. Habijan, M. Strohmaier, J. Dolenc, A. Jagodic, D. Sikošek, A. Rodič, A. Osojnik, J. Klofutar: Razvoj domače proizvodnje stelitnih zlitin

KZT 26 (1992) 1-2 , 215-218 Breskvar Bojan, B. Hertl, A. Osojnik, I. Banič-Kranjčevič: Frakcionirana kristalizacija aluminija

KZT 26 (1992) 1 - 2 , 219-222

Smajič Nijaz: Razvoj superferitnega nerjavnega jekla KZT 26 (1992) 1 -2 , 223-225

Paulin Andrej, V. Gontarev, D. Dretnik: Pridobivanje ple-menitih kovin iz sekundarnih surovin z majhnim deležem teh kovin KZT 26 (1992) 1 -2 , 226-229

Obal M., S. Rozman, G. Todorovič, S. Fajmut-Štrucelj: Pri-dobivanje Ge-preeipitata iz ZnS-koncentrata flotacije Rudnika Mežica KZT 26 (1992) 1-2 , 230-233

Obal M., S. Rozman, R. Jager, M. Kolenc, A. Osojnik: Naravni zeoliti v procesih čiščenja odpadnih voda s povečano vsebnostjo ionov kovin KZT 26 (1992) 1-2 , 234-239

Kaker Henrik: Monte Carlo simulacija v raster elektronski mikroskopiji KZT 26 (1992) 1-2 , 240-241

Ferlež R.: Uporabne lastnosti superzlitine Ravloy 4 KZT 26 (1992) 1-2 . 242-243

Uršič Viktor, M. Tonkovič-Prijanovič, R. Jud: Volumske spremembe med strjevanjem nodularne litine

KZT 26 (1992) 1-2, 244-249

Serdarevič M., F. Mujezinovič, H. Babahmetovič: Poboljšanje kvaliteta kovačkih ingota izradom djelomično de-zoksidiranog čelika u peči i dezoksidacijom u vakuumu

KZT 26 (1992) 1-2, 250-253

Pihura D.: Ut jeca j RH-postupka na smanjenje rezidual-nih i oligo elemenata u visokokvalitetnim visokougljeničnim čelicima i unapri jedjenje kvaliteta

KZT 26 (1992) 1-2, 254-254

Kert J., J. Kunstelj, M. Podgornik, J. Lamut: Razvoj ekološkega inženiringa v slovenskih železarnah

KZT 26 (1992) 1-2 , 254-254

Kejžar Rajko, U. Kejžar, M. Hrženjak, L. Kosec: Produk-tivno navarjanje visoko legiranih jekel na konstrukcijska jekla

KZT 26 (1992) 1-2 , 254-255

Kejžar Rajko, B. Kejžar, M. Hrženjak, J. Lamut, J. Sa-vanovič: Sintetični minerali v oblogi bazičnih elektrod in varil-nih praškov KZT 26 (1992) 1 -2 , 255-256

Šarler B., A. Košir, A. Križman, D. Križman: Posodobitve procesa kontinuiranega ulivanja na podlagi eksperimentalno potr jenega matematičnega modeliranja

KZT 26 (1992) 1-2, 257-257

Kejžar Rajko, M . Hrženjak: Izdelava orodij z navarjanjem v kokilo KZT 26 (1992) 1 -2 . 257-258

Kejžar Rajko, B. Kejžar: Spremembe mletih ferozlitin pri proizvodnji dodajnih materialov KZT 26 (1992) 1 -2 . 258-259

Risteski Ice B.: Dimenzioniranje meniska tokom kontinuira-nog livenja čelika KZT 26 (1992) 3, 271-274

Bratina Janez: Električni lok v obločni peči KZT 26 (1992) 3. 275-282

Bolčina Marjan: Uporaba PC preglednic s poudarkom na reševanju temperaturnih polj in polj mešanja taline

KZT 26 (1992) 3. 283-288

Smajič Nijaz: Computer Simulation and Optimization of VOD Treatment KZT 26 (1992) 4, 313-317

Vodopivec Franc: Microalloying of Steel KZT 26 (1992) 4, 319-328

Črnko Josip: The Dependence of the Heat Energy Consump-tion upon the Working Intensity and the Frequency of the Iso-lation Maintenance of a Pusher-type Furnace

KZT 26 (1992) 4, 329-331

Arzcnšek Boris, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt, G. Velikajnc: Tool-Steel Wire Drawing at Ele-vated Temperatures KZT 26 (1992) 4, 333-335

Odesskij P. 1)., V. A. Kučerenko, N. Kudajbergenov, I. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation KZT 26 (1992) 4. 337-342

Gojič M., M. Balenovič, L. Kosec, L. Vehovar, L.J. Ma-lina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrit t lement KZT 26 (1992) 4, 343-349

Jenko Monika, A. Kveder, S. Spruk, L. Koller: Chromizing of Iron KZT 26 (1992) 4, 351-355

2. Avtorsko kazalo

Arzcnšek Boris, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt, G. Velikajne: Tool-Steel Wire Drawing at Ele-vated Temperatures KZT 26 (1992) 4, 333-335 Ažman Slavko: Razvoj in problematika mikrolegiranih jekel za petrokemično industrijo KZT 26 (1992) 1 - 2 . 015-022 Bolčina Marjan: Uporaba PC preglednic s poudarkom na reševanju temperaturnih polj in polj mešanja taline

KZT 26 (1992) 3, 283-288 Božič Antonija: Problematika sežigalnih naprav

KZT 26 (1992) 1 -2 . 114-117 Bratina Janez: Električni lok v obločni peči

KZT 26 (1992) 3, 275-282 Breskvar Bojan, B. Hertl, A. Osojnik, I. Banič-Kranjčevič: Frakcionirana kristalizacija aluminija

KZT 26 (1992) 1 -2 , 219-222 Črnko Josip: The Dependence of the Heat Energy Consump-tion upon the Working Intensity and the Frequency of the Iso-lation Maintenance of a Pusher-type Furnace

KZT 26 (1992) 4, 329-331 Doberšek M., L Kosovinc: Vplivi indija na lastnosti dentalnih zlitin Ag-Pd-Au 1 -Cu-Zn5-In5 . . KZT 26 (1992) 1 -2 . 183-187 Dobi D.: Uporaba instrumentiranega Charpyja pri razvoju jekel

KZT 26 (1992) 1 -2 . 063-073 Drofenik B., F. Vodopivec, M. Jenko: Rast rekristaliziranih zm v zlitini železa in silicija mikrolegirani s selenom in kositrom KZT 26 (1992) 1 -2 , 151-155 Ferlež R.: Uporabne lastnosti superzlitine Ravloy 4

KZT 26 (1992) 1 -2 , 242-243 Gojič M., M. Balenovič, L. Kosec, L. Vehovar, L.J. Ma-lina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement KZT 26 (1992) 4. 343-349 Hajnže D., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini Fe in Si KZT 26 (1992) 1 -2 , 147-150 Iharoš B.: Izrada normativno-tehnološke karte hladnog vučenja čeličnih cijevi KZT 26 (1992) 1 - 2 , 097-099 Jenko Monika, F. Vodopivec, B. Praček: Raziskave segre-gacij na površini Fe-Si-C-Sb zlitin z metodo AES

KZT 26 (1992) 1-2 , 201-204 Jenko Monika, A. Kveder, S. Spruk, L,. Koller: Chromizing of Iron KZT 26 (1992) 4, 351-355 Kaker Henrik: Monte Carlo simulacija v raster elektronski mikroskopiji KZT 26 (1992) 1 -2 . 240-241 Kanalcc Slavko, M. Tolar, J. Lamut: Racionalizacije porabe apna v jeklarni Bela KZT 26 (1992) 1 -2 . 140-142 Kejžar Rajko: Oplemenitenje površin z navarjanjem in metalizacijo KZT 26 (1992) 1 -2 . 079-084 Kejžar Rajko. U. Kejžar, M. Hrženjak, L. Kosec: Produk-tivno navarjanje visoko legiranih jekel na konstrukcijska jekla

KZT 26 (1992) 1 -2 . 254-255 Kejžar Rajko, B. Kejžar, M. Hrženjak, J. Lamut, J. Sa-vanovič: Sintetični minerali v oblogi bazičnih elektrod in varil-nih praškov KZT 26 (1992) 1 -2 . 255-256 Kejžar Rajko, M. Hrženjak: Izdelava orodij z navarjanjem v kokilo KZT 26 (1992) 1 -2 . 257-258 Kejžar Rajko, B. Kejžar: Spremembe mletih ferozlitin pri proizvodnji dodajnih materialov KZT 26 (1992) 1 -2 . 258-259 Kcrt A., J. Apat, J. Lamut: Pretaljevanje sekundarnih surovin

KZT 26 (1992) 1 - 2 . 143-146 Kert J., J. Kunstelj, M. Podgornik, J. Lamut: Razvoj ekološkega inženiringa v slovenskih železarnah

KZT 26 (1992) 1 -2 . 254-254 Kolenko Tomaž, B. Glogovac, D. Novak, D. Žagar, B. Omejc: Konceptualna rešitev procesnega vodenja ogrevanja vložka v potisni peči KZT 26 (1992) 1 - 2 , 085-088

Koroušič Blaženko: Pomen matematičnega modeliranja pri študiju jeklarskih procesov KZT 26 (1992) 1-2 , 194-196 Kosec Ladislav, N. Igerc, B. Kosec, B. Godec, B. Urnaut: Temperaturna utrujenost j e k e l . . KZT 26 (1992) 1-2, 074-078 Kostajnšek J.: Čiščenje talin z vpihovalnim rotorjem za uva-janje čistilnih plinov v t a l i n o . . . KZT 26 (1992) 1-2, 178-180 Krajcar Josip, V. Ferketič, D. Vukovič, A. Ivančan, J. Bu-torac, A. Iharoš: Površinske greške čeličanskog izvora na bešavnim cijevima KZT 26 (1992) 1-2 , 094-096 Kristan T.: Aluminij in avtomobilska industrija

KZT 26 (1992) 1-2 , 181-182 Kurbos Mojca, J. Lamut, T. Kolenko, M. Debelak: Vpliv pogojev konti litja na lastnosti slabov

KZT 26 (1992) 1-2 , 131-133 Lamut Jakob, F. Pavlin, A. Poklukar: Taljenje vložka s plinom v jaškasti kupolni peči . KZT 26 (1992) 1-2 , 038-040 Legat Franc: Toplotna obdelava verig s poudarkom na indukciji KZT 26 (1992) 1-2 , 049-052 Lesjak D., J. Lamut, V. Gontarcv, A. Purkat, M. Debelak: Modelne raziskave v j ek la r s tvu .KZT 26 (1992) 1-2, 134-139 Leš P., F. Dover: Tehnologija valjanja radialno orebrenih cevi za prenos toplote KZT 26 (1992) 1 -2 , 089-093 Lovrečič Saražin Marko: Hamiltonski indeks grafa

KZT 26 (1992) 1-2 , 123-124 Obal M., S. Rozman, G. Todorovič, S. Fajmut-Štrucclj: Pri-dobivanje Ge-precipitata iz ZnS-koncentrata llotacije Rudnika Mežica KZT 26 (1992) 1-2 , 230-233 Obal M., S. Rozman, R. Jager, M. Kolenc, A. Osojnik: Naravni zeoliti v procesih čiščenja odpadnih voda s povečano vsebnostjo ionov kovin KZT 26 (1992) 1-2 , 234-239 Odesskij P. D., V. A. Kučerenko, N. Kudajbergenov, I. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation KZT 26 (1992) 4, 337-342 Paulin Andrej, V. Gontarev, D. Dretnik: Pridobivanje ple-menitih kovin iz sekundarnih surovin z majhnim deležem teh kovin KZT 26 (1992) 1-2 , 226-229 Pihura D.: Ut jecaj RH-postupka na smanjenje rezidual-nih i oligo elemenata u visokokvalitetnim visokougljeničnim čelicima i unapri jedjenje kvaliteta

KZT 26 (1992) 1-2 , 254 -254 PloStajner Henrik, V. Prešern, G. Todorovič: Optimizacija tehnologije izdelave in odlivanja jekla v Železarni Store

KZT 26 (1992) 1-2 , 034-037 Rcnko M., A. Osojnik: Razvoj metod za analizo redkih zemelj v zlitinah s posebnimi lastnostmi

KZT 26 (1992) 1-2 , 118-122 Rihar G.: Obdelava kovin z žarkovnimi izvori energije

KZT 26 (1992) 1-2 , 110-113 Risteski Ice B.: Dimenzioniranje meniska tokom kontinuira-nog livenja čelika KZT 26 (1992) 3, 271-274

Rodič Alenka, J. Žvokelj, F. Legat, S. Krivec: Vpliv kemi-jske sestave na lastnosti jekel za verige po toplotni obdelavi .

KZT 26 (1992) 1-2, 053-057 Rodič Jože, K. Habijan, M. Strohmaicr, J. Dolenc, A. Jagodic, D. Sikošek, A. Rodič, A. Osojnik, J. Klofutar: Razvoj domače proizvodnje stelitnih zlitin

KZT 26 (1992) 1-2 , 215-218

Serdarevič M., F. Mujezinovič, H. Babahmetovič: Poboljšanje kvaliteta kovačkih ingota izradom djelomično de-zoksidiranog čelika u peči i dezoksidacijom u vakuumu

KZT 26 (1992) 1 -2 , 250-253 Smajič Nijaz: Dinamični model vakuumske redukcije VOD žlinder KZT 26 (1992) 1 - 2 , 205-208

Smajič Nijaz: Razvoj superferitnega nerjavnega jekla KZT 26 (1992) 1 - 2 , 223-225

Smajič Nijaz: Computer Simulation and Optimization of VOD Treatment KZT 26 (1992) 4, 313-317 Smolej A.: Razvoj sodobnih zlitin aluminija

KZT 26 (1992) 1 - 2 . 172-177

Šarler B., A. Košir, A. Križman, D. Križman: Posodobitve procesa kontinuiranega ulivanja na podlagi eksperimentalno potrjenega matematičnega modeliranja

KZT 26 (1992) 1 - 2 , 257-257

Segel Jože: Računalniška podpora krmiljenju p r o i z v o d n j e . . . KZT 26 (1992) 1 - 2 , 188-193

Stok B., N. Mole: Numerična simulacija procesa izdelave in-gotov po EPŽ postopku KZT 26 (1992) 1 - 2 , 197-200

Šuštaršič Borivoj, Z. Lengar, S. Tašner, J. Holc, S. Be-seničar: Izdelava AlNiCo trajnih magnetov iz vodno atom-iziranih prahov KZT 26 (1992) 1 - 2 , 104-109

Šuštaršič Borivoj, M. Torkar, M. Jenko, B. Breskvar, F.Vodopivec: Procesi atomizacije kovinskih gradiv in konsol-idacije kovinskih prahov — II. del

KZT 26 (1992) 1 - 2 , 209-214 Tehovnik F., B. Koroušič, V. Prešern: Optimizacija modi-fikacije nekovinskih vključkov v jeklih obdelanih s Ca

KZT 26 (1992) 1 -2 , 125-130 Tolar Miha, J. Lamut: Racionalizacija in optimiranje proizvodnje v jeklarni Bela . . . . KZT 26 (1992) 1 - 2 , 023-029

Torkar Matjaž, B. Šuštaršič: Preiskava vodno atomiziranega prahu iz zlitine Nimonic 80A . . KZT 26 (1992) 1 - 2 , 100-103

Triplat Jože: Razvoj tehnologije izdelave nerjavnih jekel v Železarni Jesenice od leta 1984 naprej

KZT 26 (1992) 1 -2 , 030-033 Uršič Viktor, M. Tonkovič-Prijanovič, R. Jud: Volumske spremembe med strjevanjem nodularne litine

KZT 26 (1992) 1 - 2 , 244-249 Vehovar Leopold, M. Mavhar: Korozijska odpornost ner-javnega superferitnega jekla Acrom 1 super v primerjavi z avstenitnim nerjavnim jeklom Acroni 11 Ti

KZT 26 (1992) 1 - 2 , 156-164

Vehovar Leopold, M. Pečnik: Korozijska odpornost superzl-iline Ravloy 4 KZT 26 (1992) 1 - 2 , 165-168

Vehovar Leopold, T. Pavlin: Prepustnost mikrolegiranih jekel za vodik KZT 26 (1992) 1 - 2 , 169-171

Vižintin J.: Razvoj in pomen tribologije doma in v svetu . . . KZT 26 (1992) 1 - 2 , 041-048

Vodopivec Franc: Microalloying of Steel KZT 26 (1992) 4, 319-328

Vojvodič-Gvardjančič Jelena: Nizkotemperatuma meja uporabnosti mikrolegiranih jekel s stališča lomne mehanike . .

KZT 26 (1992) 1 - 2 , 058-062

Contents

N. Smajič

Computer Simulation and Optimization of VOD Treatment

KZT, 26 (1992) 4, p 313-317

Mathematical model and the software CAPSS (Computer Aided Produc tion of Stainless Steel) developed as a part of URP-C2-2566 research program were used for computer simulation of EAF-VOD-CC stainless steelmaking technology line. Basic aim of model testing was to optimise VOD treatment vvith emphasis on obtaining the maximum productivity at the lowest possible thermal load of VOD ladle and EAF tap temperature. It vvas concluded that only computer controlled oxygen blowing can secure maximum productivity at the acceptable thermal load of VOD ladle and lowest EA furnace tap temperature.

Author's Abstract

Microalloying, microstructure, mechanical properties, controlled rolling, pre-cipitation

F. Vodopivec

Microalloying of Steel

KZT, 26 (1992) 4, 'p 319-328

The article is a revievv on the effects of microalloying on steel properties. The follovving topics are deseribed: austenite grain size and homogenetic, pre-cipitation hardening, precipitation processes during the hot vvorking and the inhibition of static recrystallisation of austenite, yield stress, noteh toughness and transition temperature brittle-ductile fracture, interaetion of elements in precipitates, controlled rolling and forging, rolling vvith controlled recrystalli-sation and the economy of microalloying of structural steels vvith aluminium, niobium, vanadium and titanium.

Author 's Abstract

J. Čmko

The Dependence of the Heat Energy Consumption upon the Working Intensitv and the Frequency of the Isolation Maintenance of a Pusher-type Furnace

KZT, 26 (1992) 4, p 329-331

The paper presents results of the analysis of the influence of the vvorking intensity and isolation maintenance frequency increase on the example of a pusher-type furnace in a strip and billet rolling mili. The results obtained analytically show that the specific heat energy consumption decreases for about 18% if the intensity of the pusher-type furnace inereases 1.81 times. The results obtained analysing operating data of the furnace work do not show significant deviations from the results obtained analytically. The same way, the increase of the isolation maintenance frequency and that of the layers removal from the floor to the half of the period (6 mths, instead of 12 mths) would decrease the average specific heat energy consumption for about 10%. The increase of the pusher-type furnace maintenance frequency can be realized successfully by better month and vveek planning of rolling, vvhereas bringing of the coefficient of capacity utilization of the furnace to normal limits (0.85-0.95) is not possible only by interventions in planning.

Author's Abstract

Steel workability, tool steels, vvire dravving at elevated temperatures

B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt

Tool-steel VVire Dravving at Elevated Temperatures

KZT, 26 (1992) 4, p 333-335

Tool steels transfer mostly tvvo dravvs at eold dravving, therefore the dravv-ing technology at elevated temperatures vvas developed. The dravving abilities of BRM2 tool steel at temperatures up to 700° C and dravving device for dravv-ing at elevated temperatures, developed at our institute in cooperation vvith Steel Plant Ravne, vvas deseribed in this vvork. The aim at development of the technology vvas to use eold vvire dravving devices as much as possible to cheapen the technology and enable the technology transfer in industrial produetion.

Author 's Abstract

P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič

Resistance of Structural Steel to Crack Formation and Propagation

KZT, 26 (1992) 4, p 337-342

Parameters of LEFM can be a measure for seleetion of steel with various strengths and yield strengths. They are applicable only if they are measured in conditions of constrained plastic deformation. This can be achieved by an influence of corrosion medium or vvith impact tests. These parameters are closely interrelated vvith the steel micro strueture and purity. They are suitable for designing struetures resistant to brittle fracture if they succeed to enclose the operational conditions of the strueture.

Author's Abstract

M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina

Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement

KZT, 26 (1992) 4, p 343-349

Various types of damages on pipes used in oil and gas produetion are deseribed. The pipes are made of low-alloyed manganese- vanadium steel. Not heat-treated steel pipes are very sensitive to hydrogen embrittlement (embrittlement index 86%). Hardening and tempering highly improves the resistivity of pipes to hydro gen embrittlement (embrittlement index 25%) vvhich is also inter related to the fracture mechanism in steel.

Author 's Abstract

Contents

Chromium diffusion, chromizing of iron, vacuum chro mizing of iron, minia-ture hermetic relays, I e - C r layers resis tive to corrosion media, coercitivity

M. Jenko, A. Kveder, S. Spruk, L. Koller

Chromizing of Iron

KZT, 26 (1992) 4, p 351 -355

The theoretical aspects of CVD processes of iron chromizing and the comparison with PVD process are presented. It is shown that nonc of CVD processes is suitable for the proteetion of iron parts for magnetic circuit in miniature hermetic relays. For this purpose PVD process of vacuum chromiz-ing was developed. With this procedure the maximal chromium content of 15% Cr at the surface was obtained. Such chromium layer assures the optimal magnetic properties, a very low coercitivity and a good weldability.

Author's Abstract

Vsebina

N. Smajič

Računalniška simulacija in optiniiranje VOD obdelave

KZT, 26 (1992) 4, s 313-317

V okviru petletnega raziskovalnega programa URP-C2-2566 izdelani model in računalniški program CAPSS (Computer Aided Production of Stainless Steel) je bil uporabljen za računalniško simulacijo EOP-VOD-KL tehnologije za izdelavo neijavnih jekel. Osnovni namen modelnih poskusov je bil optimiranje V O D obdelave s posebnim poudarkom na zagotavljanju maksimalne produktivnosti V O D naprave ob najmanjši možni toplotni obre-menitvi V O D ponovce in minimalni temperaturi preboda. Ugotovili smo, da le računalniško programirano pihanje zagotavlja maksimalno produktivnost ob še sprejemljivi toplotni obremenitvi VOD ponovce in minimalni temper-aturi preboda.

Avtorski izvleček

F. Vodopivec

Mikrolegiranje jekla

KZT, 26 (1992) 4, s 319-328

Mehanizmi vpliva mikrolegiranja na trdnostne lastnosti in žilavost jekla. Vpliv na velikost zrn, izločilno utrditev, procesi vroče deformacije in gospo-darnost mikrolegiranja z Al, Nb, V in Ti.

Avtorski izvleček

J. Črnko

Ovisnost utroška toplinske energije od inteziteta rada i učestalosti održavanja izolacije potisne peči

KZT, 26 (1992)4 , s 329-331

U radu su prikazani rezultati analize utjecaja povečanja intenziteta rada i učestalosti održavanja izolacije na primjeru potisne peči u valjaonici traka i gredica. Rezultati dobiveni analitičkim putem pokazuju da se specifični utrošak toplinske energije smanji za oko 18% ako se poveča intenzitet rada po-tisne peči 1.81 puta. Značajna odstupanja od rezultata dobivenih analitičkim putem ne pokazuju ni rezultati dobiveni analizom pogonskih podataka o radu potisne peči. Isto tako, povečanje učestalosti održavanja izolacije i čiščenja poda od nastalih naslaga na polovicu vremena (6 mjeseci) od dosadašnjeg (12 mjeseci) smanjilo bi prosječni specifični utrošak toplinske energije za oko 10%. Povečanje učestalosti održavanja potisne peči moguče je uspješno ostvariti boljim mejsečnim i tjednim planovima valjanja, dok dovo—enjdco-eficijenta iskorištenja kapaciteta potisne peči u normalne granice (0.85-0.95) nije moguče samo zahvatima u planove valjanja ostvariti.

Avtorski izvleček

Preoblikovanje jekel, orodna jekla, vlečenje žice pri povišanih temperaturah

B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt

Vlečenje žice iz orodnih jekel pri povišanih temperaturah

KZT, 26 (1992) 4, s 333-335

Orodna jekla prenesejo pri vlečenju v hladnem stanju največ dva vleka, zato smo razvili tehnologijo vlečenja jekel pri povišanih temperaturah. V delu smo opisali vlečne sposobnosti jekla BRM2 pri temperaturah do 700° C in linijo za vlečenje žice, ki smo jo razvili na Inštitutu za kovinske materiale in tehnologije v Ljubljani v sodelovanju s sodelavci iz Železarne Ravne. Cilj pri razvoju tehnologije je bil uporabiti čim več opreme, ki jo uporabljamo tudi pri hladnem vlečenju, kar tehnologijo poceni in omogoča njen hitrejši prenos v proizvodnjo.

Avtorski izvleček

P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič

Odpornost gradbenih jekel proti nastanku in širjenju razpoke

KZT, 26 (1992) 4, s 337-342

Parametri LEFM so lahko merilo za selekcijo jekel z različno trdnostjo ali napetostjo tečenja. Uporabni so le, če so izmerjeni v pogojih zelo omejene plastične deformacije. To se lahko doseže z vplivom korozijskega medija ali pri udarnih preizkusih. Ti parametri so tesno povezani z mikrostrukturo jekla in njegovo čistostjo. Primeri so za izračun konstrukcij odpornih proti krhkemu lomu, če uspejo zapopasti pogoje pri uporabi objektov.

Avtorski izvleček

M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina

Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti

KZT, 26 (1992) 4, s 343-349

Opisane so različne oblike poškodb cevi za pridobivanje nafte in zemeljskega plina iz malolegiranega mangan-vanadijevega jekla. Cevi iz toplotno neobdelanega jekla so zelo občutljive na vodiko vo krhkost (in-deks krhkosti 86%), s poboljšanjem jekla pa se odpornost cevi proti vodikovi krhkosti zelo poveča (indeks krhkosti 25%), kar je povezano tudi z mehaniz-mom prelomov jekla.

Avtorski izvleček

Vsebina

Difuzija kroma, difuzijsko kromanje, vakuumsko kromanje, miniaturni her-

metični releji, Fe-Cr plasti odporne proti koroziji, koercitivnost

M. Jenko, A. Kveder, S. Spruk, L. Koller

Difuzijsko kromanje železa

KZT, 26 (1992) 4, s 351-355

Podani so teoretični vidiki CVD postopkov difnzijskega kromanja železa v primerjavi s PVD postopkom. Raziskava CVD postopkov je pokazala, da za zaščito sestavnih delov železnega magnetnega kroga miniaturnih her-metičnih relejev ni primeren nobeden izmed le-teh. Zato so avtorji razvili PVD postopek - tehnologijo vaku umskega kromanja. S tem postopkom je dosežena maksimalna končen tracija kroma na povriSini 15% Cr, kar je dovolj, da so vzorci korozijsko obstojni v medijih, ki jih predpisujejo standardi. S PVD postopkom dosežemo optimalne magnetne lastnosti (nizka koercitivnost) in dobro varivost.

Avtorski izvleček

. n O

debelo, srednjo in tanko pločevino

hladno valjane trakove in pločevino

dinamo trakove in pločevino

nerjavne trakove in pločevino

vlečeno, brušeno in luščeno jeklo

valjano in vlečeno žico

patentirano žico

pleteno patentirano žico za prednapeti beton

hladno oblikovane profile

kovinske podboje za vrata

dodajni material za varjenje

žičnike

tehnične pline

STORITVE

prevaljanja, vlečenja, iztiskanja

in toplotne obdelave

pločevin in žice

tehnične dejavnosti: elektro, strojne,

konstrukcijske, obrtne in tehnične

I Z D E L U J E

• M I K R O L E G I R A N A J E K L A • N E R J A V N A J E K L A • E L E K T R O P L O Č E V I N E IN T R A K O V E

• v r o č e v a l j a n e t r a k o v e in p l o č e v i n e • h l a d n o v a l j a n e t r a k o v e in p l o č e v i n e • d i n a m o t r a k o v e in p l o č e v i n e • n e r j a v n e t r a k o v e in p l o č e v i n e • h l a d n o o b l i k o v a n e prof i le • k o v i n s k e p o d b o j e z a v r a t a

• N U D I M O T U D I S T O R I T V E

• p r e v a l j a n j a , i z t iskan ja , k r o j e n j a in t o p l o t n e o b d e l a v e p l o č e v i n

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